The Iraqi Army has entered Mosul for the first time in over two years at the start of a battle which is likely to end in a decisive defeat for Isis. The significance of the fight for Mosul will be all the greater for Isis because its self-declared caliph, Abu Bakr al-Baghdadi, is believed to be still inside the city, a senior Kurdish official has told The Independent.

Fuad Hussein, chief of staff to Kurdish President Massoud Barzani, said in an exclusive interview that his government had information from multiple sources that “Baghdadi is there and, if he is killed, it will mean the collapse of the whole [Isis] system.” Isis would have to choose a new caliph in the middle of a battle, but no successor would have the authority and prestige of Baghdadi, the leader who surprised the world by establishing the caliphate after capturing Mosul in June 2014.

Baghdadi has kept himself concealed for the last eight or nine months according to Mr Hussein, who added that the caliph had become very dependent on Isis commanders from Mosul and Tal Afar, a city just to the west of Mosul. Other senior and better known figures in Isis, particularly those from Syria and other countries, have been killed since their initial triumphs in the summer of 2014 when they took over much of northern Iraq and eastern Syria.

The presence of Baghdadi in Mosul may complicate and prolong the battle for Mosul as his surviving adherents fight to the death to defend him. Mr Hussein said that “it is obvious that they will lose, but not how long this will take to happen.” He said that Kurdish Peshmerga forces had been impressed by the extraordinary number of tunnels that Isis had dug in order to provide hiding places in the villages around Mosul.

Iraqi Special Forces advanced into Mosul, which once had a population of two million, on Tuesday seizing the state television on the east bank of the Tigris River that divides the city in half. Mr Hussein said that the speed of the fall of Mosul would depend on many factors especially whether or not Isis “is going to destroy the five bridges over the river.”

Baghdadi speaking in 2014 (AP)

Iraqi army units backed by US-led air strikes have been attacking across the Nineveh Plain to the east of Mosul, capturing empty towns and villages from which the inhabitants have almost entirely fled. Where Christians and other minorities have tried to return to their old homes in towns like Bartella and Qaraqosh, they have found them looted and often burned by retreating Isis fighters.

Iraqi troops entered Gogjali, a district inside Mosul's city limits, and later the borders of the more built-up Karama district, according to Major General Sami al-Aridi of the Iraqi special forces. Under an agreement reached before the offensive began on 17 October, Kurdish Peshmerga and Shia-militia paramilitaries known as the Hashd al-Shaabi, will not join the attack into Mosul, which is a largely Sunni Arab city.

In pictures: Mosul offensive Show all 40 1 /40 In pictures: Mosul offensive In pictures: Mosul offensive A doctor carries an Iraqi newborn baby at a hospital in Mosul, Iraq July 18, 2017. Reuters In pictures: Mosul offensive Iraqi girls play at a yard of a school in Mosul, Iraq July 18, 2017alal Reuters In pictures: Mosul offensive A woman on crutches who is a relative of men accused of being Islamic State militants is seen at a camp in Bartella, east of Mosul, Iraq July 15, 2017. Picture taken July 15, 2017. Reuters In pictures: Mosul offensive A displaced girl, who fled from home carries a doll at Hamam al-Alil camp south of Mosul, Iraq July 13, 2017. Reuters In pictures: Mosul offensive Iraqi federal police members and civilians celebrate in the Old City of Mosul on 9 July 2017 after the government's announcement of the "liberation" of the embattled city. Iraqi Prime Minister Haider al-Abadi's office said he was in "liberated" Mosul to congratulate "the heroic fighters and the Iraqi people on the achievement of the major victory" AFP/Getty Images In pictures: Mosul offensive A picture taken on 9 July 2017, shows a general view of the destruction in Mosul's Old City. Iraq will announce imminently a final victory in the nearly nine-month offensive to retake Mosul from jihadists, a US general said Saturday, as celebrations broke out among police forces in the city. AFP In pictures: Mosul offensive Members of the Iraqi federal police raise the victory gesture as they ride on a humvee while advancing through the Old City of Mosul on 28 June 2017, as the offensive continues to retake the last district held by Islamic State (IS) group fighters. AFP/Getty Images In pictures: Mosul offensive Smoke billows as Iraqi forces advance through the Old City of Mosul on 26 June 2017, during the ongoing offensive to retake the last district held by the Islamic State (IS) group. AFP/Getty Images In pictures: Mosul offensive An Iraqi man wearing the green scarf of the Shi'ite faith kisses an Iraqi Army soldier on safely reaching the Iraqi forces position as Iraqi civilians flee the Old City of west Mosul where heavy fighting continues on 23 June 2017. Iraqi forces continue to encounter stiff resistance with improvised explosive devices, car bombs, heavy mortar fire and snipers hampering their advance. Getty Images In pictures: Mosul offensive A picture taken from the inside of an Iraqi forces armoured vehicle shows residents walking through a damaged street as troops advance towards Mosul's Old City on 18 June 2017, during the ongoing offensive to retake the last district still held by the Islamic State (IS) group. Military commanders told AFP the assault had begun at dawn after overnight air strikes by the US-led coalition backing Iraqi forces. They said the jihadists were putting up fierce resistance. AFP/Getty Images In pictures: Mosul offensive Iraqi Army soldiers advance in a destroyed street after an Iraqi forces airstrike targeted an Islamic State sniper position 17 June 2017 in al-Shifa, the last district of west Mosul under Islamic State control. IS snipers, as well as car and suicide bomb attacks continue to hinder the Iraqi forces efforts to retake the final district. A series of airstrikes by Iraqi helicopter gunships attempted to hit multiple Islamic State sniper positions in al-Shifa. Getty Images In pictures: Mosul offensive An Iraqi soldier frisks a displaced Iraqi man at a temporary camp in the compound of the closed Nineveh International Hotel in Mosul on 16 June 2017 which was recovered by Iraqi troops from Islamic State group fighters earlier in the year. A screening centre set up in the compound's fairgrounds sees a constant stream of Iraqis fleeing the battle for Mosul, awaiting their turn to be checked by the Iraqi forces who are searching for suspected Islamic State (IS) group members. The small fairground lies at the end of a pontoon bridge across the Tigris recently opened to civilians that is the only physical link between the two banks of the river. AFP/Getty Images In pictures: Mosul offensive Iraqis staying at the al-Khazir camp swim in a river near the camp for internally displaced people, located between Arbil and Mosul on 11 June 2017. AFP/Getty Images In pictures: Mosul offensive Iraqi government forces drive on a road leading to Tal Afar on 9 June 2017, during ongoing battles to retake the city from Islamic State (IS) group fighters. AFP/Getty Images In pictures: Mosul offensive An Iraqi policeman carries a poster bearing an image of Mosul's iconic leaning minaret, known as the "Hadba" (Hunchback), on 22 June 2017. AFP/Getty Images In pictures: Mosul offensive Iraqis stand in line to receive food aid in western Mosul's Zanjili neighbourhood on 7 June 2017, during ongoing battles as Iraqi forces try to retake the city from Islamic State (IS) group fighters. Living conditions in Mosul have again deteriorated since the start of the Iraqi government's offensive on the city in October in which they retook a large part of the west of the city. AFP/Getty Images In pictures: Mosul offensive Displaced Iraqis carry lightbulbs and sacks as they evacuate from western Mosul's Zanjili neighbourhood as government forces advance in the area during their ongoing battle against Islamic State (IS) group fighters on 13 May 2017 AFP/Getty Images In pictures: Mosul offensive A member of the Iraqi Counter-Terrorism Service (CTS) flashes the victory gesture as he patrols in western Mosul's al-Islah al-Zaraye neighbourhood on 13 May 2017 AFP/Getty Images In pictures: Mosul offensive Iraqi army soldiers from the 9th armoured division on a truck flash the sign of victory as they drive back from Mosul to the town of Qaraqosh (also known as Hamdaniya) Getty In pictures: Mosul offensive Members of Iraqi forces flash the sign of victory on their vehicle as they advance towards Hammam al-Alil area south of Mosul Getty In pictures: Mosul offensive A member of Iraqi security forces gestures in Hammam al-Alil, south of Mosul, Iraq Reuters In pictures: Mosul offensive Iraqi children, one flashing the sign of victory, greet Iraqi army's soldiers from the 9th armoured division in the area of Ali Rash, adjacent to the eastern Al-Intissar neighbourhood of Mosul Getty In pictures: Mosul offensive Peshmerga forces look at a tunnel used by Islamic State militants near the town of Bashiqa, east of Mosul, during an operation to attack Islamic State militants in Mosul, Iraq Reuters In pictures: Mosul offensive An Iraqi soldier takes a photograph with his phone as his comrade stands next to a detained man, whom the Iraqi army soldiers accused of being an Islamic State fighter, who was fleeing with his family in the Intisar disrict of eastern Mosul, Iraq Reuters In pictures: Mosul offensive Iranian Kurdish female members of the Freedom Party of Kurdistan (PAK) hold a position in an area near the town of Bashiqa, some 25 kilometres north east of Mosul Getty In pictures: Mosul offensive Iraqi families, who fled their homes in Hamam al-Alil, gather on the outskirts of their town Getty In pictures: Mosul offensive Displaced people walk past a checkpoint near Qayara, south of Mosul, Iraq AP In pictures: Mosul offensive Iraqi families who were displaced by the ongoing operation by Iraqi forces against jihadists of the Islamic State group to retake the city of Mosul, are seen gathering in an area near Qayyarah In pictures: Mosul offensive A boy who just fled Abu Jarbuah village is seen with his family at a Kurdish Peshmerga position between two front lines near Bashiqa, east of Mosul, Iraq Reuters In pictures: Mosul offensive An Iraqi child eats a pomegranate upon the arrival of Iraqi forces in the village of Umm Mahahir, south of Mosul Getty In pictures: Mosul offensive People who just fled Abu Jarbuah village sit as they eat at a Kurdish Peshmerga position between two front lines near Bashiqa, east of Mosul, Iraq Reuters In pictures: Mosul offensive A couple who just fled Abu Jarbuah village are escorted by Kurdish Peshmerga soldiers Reuters In pictures: Mosul offensive Women carry a boy over a wall as civilians flee their houses in the village of Tob Zawa, Iraq AP In pictures: Mosul offensive An Iraqi soldier and a civilian ride a motorbike as smoke rises behind them, on the road between Qayyarah and Mosul Getty In pictures: Mosul offensive A member of Iraqi forces, wearing a skull mask, waits at a checkpoint for people fleeing the main hub city of Mosul Getty In pictures: Mosul offensive An Iraqi soldier sits at a checkpoint in an area near Qayyarah Getty In pictures: Mosul offensive Iraqi men prepare food portions for Iraqi forces deployed in areas south of Mosul Getty In pictures: Mosul offensive Iraqi forces celebrate upon the arrival of vehicles bringing food to them Getty In pictures: Mosul offensive Iraqi childen smoke cigarettes upon the arrival of Iraqi forces in the village of Umm Mahahir, south of Mosul Getty In pictures: Mosul offensive A member of Iraqi forces distributes drinks to children in the village of Umm Mahahir, south of Mosul Getty

As night fell, a sandstorm blew up cutting visibility to only 100 yards making air support for Iraqi forces more difficult and bringing the fighting to an end. “Daesh (Isis) is fighting back and have set up concrete blast walls to block off the Karama neighbourhood and [stop] our troops' advance,” General Aridi said. He added later that the troops had taken the nearby state television building, the only one in Nineveh province, but there had been heavy fighting when they tried to move further into built-up areas. They are still some six miles from the city centre.

The anti-Isis offensive is dependent on US-led air strikes and the presence of US special forces. “I assure you that the Iraqi Army and the Peshmerga do not move one millimetre forward without American permission and coordination,” said one Kurdish observer. He did not think that the battle for Mosul would necessarily go on a long time. But it is increasingly difficult for the 3,000 to 5,000 Isis fighters in Mosul and the 1,500 to 2,500 on the outskirts to escape, even if they wanted to. The Iraqi Army and the Peshmerga encircle the city to the north, east and west and the Hashd are moving in from the west, cutting the last routes to Syria.

US spokesman Colonel John Dorrian said that the US-led airpowers had noticed that Isis forces could no longer move in large numbers. “And when we see them come together where there are significant numbers we will strike them and kill them,” he said during a televised press conference. Some 1,792 Iraqis of whom 1,120 were civilians were killed in October according to the UN, though the total probably does not include Isis fighters.

Eyewitnesses inside Mosul, where Isis is reported to have killed 40 Iraqi prisoners at the weekend and thrown their bodies into the Tigris, say there are few fighters to be seen in the streets. “There are mostly just teenagers with guns,” said one Mosul resident reached by telephone. Part of the city is shrouded in smoke because of air strikes and artillery fire, but also because Isis fighters are lighting fires to produce a smokescreen which will make observation from the air more difficult.

Dramatic footage shows Elite Iraq forces battle on road to Bartella in fight for Mosul

It has been reported that Isis commanders were divided on whether or not it was better for them to make a last stand in Mosul or withdraw, after inflicting the maximum number of casualties on its enemies, and revert to guerrilla warfare. Last month 100 Isis fighters staged a spectacular raid on the Kurdish-held oil city of Kirkuk. An advantage for Isis in fighting in Mosul is that it would be more difficult for the US and its allies to carry out air strikes because there may be up to 1.5 million civilians still in the city. Isis has been preventing them leaving though the number is increasing as the anti-Isis forces move forward and it becomes clear that they intend to assault the city.

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Isis has never been popular in Mosul according to local residents who detest its extreme violence, religious bigotry and subjugation of women. But it found more support in Sunni Arab villages around the city and among the Sunni Turkman of the nearby city of Tal Afar, who have always been notorious for their religious extremism and hatred for Shia and Kurds. Some observers believe that Isis might want to fight here against the Shia paramilitaries of the Hashd, because the US-led air coalition has not been providing air cover for the Hashd on the grounds that they are sectarian and under Iranian influence.