The White House is likely to delay the withdrawal of the first large phase of combat troops from Iraq for at least a month after escalating bloodshed and political instability in the country.

General Ray Odierno, the US commander, had been due to give the order within 60 days of the general election held in Iraq on 7 March, when the cross-sectarian candidate Ayad Allawi edged out the incumbent leader, Nouri al-Maliki.

American officials had been prepared for delays in negotiations to form a government, but now appear to have balked after Maliki's coalition aligned itself with the theocratic Shia bloc to the exclusion of Allawi, who attracted the bulk of the minority Sunni vote. There is also concern over interference from Iraq's neighbours, Iran, Turkey and Syria.

Late tonight seven people were killed and 22 wounded when a car bomb planted outside a cafe exploded in Baghdad's Sadr City, a Shia area, police and a source at the Iraqi interior ministry said.

The latest bomb highlights how sectarian tensions are rising, as al-Qaida fighters in Iraq and affiliated Sunni extremists have mounted bombing campaigns and assassinations around the country.

The violence is seen as an attempt to intimidate all sides of the political spectrum and press home the message to the departing US forces that militancy remains a formidable foe.

Odierno has kept a low profile since announcing the deaths of al-Qaida's two leaders in Iraq, Abu Omar al-Baghdadi and Abu Ayub al-Masri, who were killed in a combined Iraqi-US raid on 18 April. The operation was hailed then as a near fatal blow against al-Qaida, but violence has intensified ever since.

All US combat forces are due to leave Iraq by 31 August, a date the Obama administration is keen to observe as the president sends greater reinforcements to fight the Taliban in Afghanistan – a campaign he has set apart from the Iraq war, by describing it as "just".

Iraqi leaders remain adamant that combat troops should leave by the deadline. But they face the problem of not having enough troops to secure the country if the rejuvenated insurgency succeeds in sparking another lethal round of sectarian conflict.

"The presence of foreign forces sent shock waves through Iraqis," said Hoshyar Zebari, the foreign minister. "And at the beginning it was a terrifying message that they didn't dare challenge. But then they got emboldened through terrorism and acts of resistance. And as the Americans are leaving, we are seeing more of it."

Zebari said Iraq's neighbours were taking full advantage of the political stalemate.

He also hinted that they may be directly backing the violence.

"They too have been emboldened, because we haven't been able to establish a viable unified government that others can respect," he said.

"In one way or another, Iran, Turkey and Syria are interfering in the formation of this government.

"There is a lingering fear [among some neighbouring states] that Iraq should not reach a level of stability. The competition over the future of Iraq is being played out mostly between Turkey and Iran. They both believe they have a vested interest here."

The withdrawal order is eagerly awaited by the 92,000 US troops still in Iraq – they mostly remain confined to their bases. This month Odierno was supposed to have ordered the pullout of 12,500, a figure that was meant to escalate every week between now and 31 August, when only 50,000 US troops are set to remain – all of them non-combat forces.

US patrols are now seldom seen on the streets of Baghdad, where the terms of a security agreement between Baghdad and Washington are being followed strictly: this relegates them to secondary partners and means US troops cannot leave their bases without Iraqi permission.

US commanders have grown accustomed to being masters of the land no longer, but they have recently grown increasingly concerned about what they will leave behind.

Zebari said: "The mother of all mistakes that they made was changing their mission from liberation to occupation and then legalising that through a security council resolution."

Earlier this week, Allawi warned that the departing US troops had an obligation enshrined in the security agreement and at the United Nations security council to safeguard Iraq's democratic process. He warned of catastrophic consequences if the occupation ended with Iraq still politically unstable.