Image copyright AFP Image caption Nigerian government troops have been unable to stop Boko Haram militants from taking over several towns 
 
 Two female suicide bombers struck a crowded market in north-east Nigeria on Sunday, killing at least four other people and injuring dozens. 
 
 An eyewitness in the city of Potiskum, in Yobe state, told the BBC that the second bomb went off shortly after the first, hitting those helping victims. 
 
 On Saturday, a bomb strapped to a girl aged around 10 killed at least 19 people in Maiduguri, in Borno state. 
 
 The attacks come amid an offensive by Boko Haram militants in the region. 
 
 A doctor at the scene in Potiskum said he saw the remains of suicide belts on the two women, who looked to be in their early 20s. 
 
 A security official told AFP news agency that it appeared the bombs had been remote-controlled. 
 
 "The second bomber was terrified by the explosion and she tried to dash across the road but she also exploded," he said. 
 
 No group has said it carried out the attack but the BBC's Will Ross in Lagos says Islamist group Boko Haram have used female suicide bombers in the past. 
 
 The violence in north-east Nigeria has been relentless in recent weeks, our correspondent adds. 
 
 Last week, people fleeing the town of Baga said several hundred people were killed there by Boko Haram gunmen in recent days. 
 
 Scores of bodies from that attack - described by Amnesty International as possibly the "deadliest massacre" in the history of Boko Haram - are reported to remain strewn in the bush. 
 
 'Constant violence' 
 
 The UN children's agency said reports from survivors of the Baga attack and the use of a young girl as a suicide bomber in Maiduguri "should be searing the conscience of the world". 
 
 "Words alone can neither express our outrage nor ease the agony of all those suffering from the constant violence in northern Nigeria," UNICEF executive director Anthony Lake said in a statement. 
 
 "But these images of recent days and all they imply for the future of Nigeria should galvanise effective action. For this cannot go on." 
 
 Boko Haram militants have been fighting to establish an Islamic caliphate in the north-eastern states of Borno, Yobe and Adamawa, which have borne the worst violence in their five-year insurgency. 
 
 The group has taken control of many towns and villages in north-eastern Nigeria over the past year. 
 
 The conflict has displaced at least 1.5 million people, while more than 2,000 were killed last year. 
 
 Boko Haram at a glance 
 
 Image copyright AFP 
 
 Founded in 2002 
 
 Initially focused on opposing Western education - Boko Haram means "Western education is forbidden" in the Hausa language 
 
 Launched military operations in 2009 to create an Islamic state 
 
 Thousands killed, mostly in north-eastern Nigeria - also attacked police and UN headquarters in capital, Abuja 
 
 Some three million people affected 
 
 Declared terrorist group by US in 2013 
 
 Who are Boko Haram? 
 
 Profile: Boko Haram leader Abubakar Shekau 
 
 Why Nigeria has not defeated Boko Haram 
 
 Nigeria's female bombers ||||| Story highlights The girl's age was not immediately clear; guesses vary from 10 to 18 
 
 Police says explosives on her body detonated at market, killing 20 and injuring 18 
 
 The explosives went off as she was being screened at entrance of the market 
 
 Kano, Nigeria (CNN) Explosives strapped to a girl detonated at a crowded marketplace in Nigeria, killing at least 20 people and injuring 18 others, according to police. 
 
 The police estimate differed from the local hospital's estimate, which counted 51 people with blast injuries. 
 
 The blast happened at the main entrance of the ever-crowded Monday Market, the largest in the city of Maiduguri. About ‎12:40 p.m. Saturday, the girl was stopped for screening by vigilantes who man the gates of the market, according to witnesses and rescuers. 
 
 The girl was among the dead, said Gideon Jubril, Borno state's police spokesman. Her precise age was not immediately clear. 
 
 A source with inside knowledge of security operations described her as being about 17 or 18. Abur Babakura, who was at the scene, said she appeared to be about 10. 
 
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– A girl was stopped by authorities at the entrance to a Nigerian market before explosives she was carrying killed her and 19 others yesterday, an official tells CNN. The bombs were attached to the girl, whose age isn't clear, CNN reports; estimates have put her as young as 10 or as old as 18. Boko Haram militants are suspected in the Maiduguri attack; a source to the news organization says the militants capture young people to "do their dirty work." The group is said to be responsible for up to 2,000 deaths in recent weeks. Today, violence continued, with two female suicide bombers killing at least four at another market, the BBC reports.