Barack Obama is under increasing pressure to ask Iraqi Prime Minister Nouri al-Maliki to step aside, as a condition of providing US military assistance to counter Sunni Islamist insurgents storming the country.

Republican Senator John McCain said the US must step in and carry out air strikes in Iraq, but added that the president should press for the removal of Maliki from office.

Obama should "make it very clear to Maliki that his time is up", McCain said.

Senate Republican leader Mitch McConnell said the president did not intend to seek Congress's approval for military action in Iraq. The White House clarified that Barack Obama did not rule out the possibility of seeking a Congress vote on military involvement.

Iraq has formally requested the US to carry out air strikes on the militants who made further advances in Salahuddin and Diyala provinces and were closing in on the capital Baghdad, after launching an audacious campaign ten days ago with the capture of Mosul.

On Wednesday, General Martin Dempsey, chairman of the US military's Joint Chiefs of Staff, hinted that the US was in no rush to intervene militarily in Iraq. Speaking at a congressional hearing, Dempsey said it was necessary to address the chaos on the ground before considering air strikes on the militants.

Defence Secretary Chuck Hagel backed McCain's demand, though the administration has not openly called for the removal of Maliki, who was installed in power in 2006 with US blessing.

"This current government in Iraq has never fulfilled the commitments it made to bring a unity government together with the Sunnis, the Kurds and the Shia," Chuck said.

White House spokesman Jay Carney said Maliki was not a big success in governing "inclusively and that has contributed to the situation and the crisis that we have today in Iraq".

Maliki has been universally blamed for failing to heal the division between Shia and Sunni sects in the aftermath of the execution of former dictator Saddam Hussein.

The nearly six million Sunni people in Iraq feel they have been marginalised and treated like second-class citizens under the rule of Maliki's Shia-led government.

Though the US was instrumental in putting Maliki in power, he was recently been seen as gravitating to fellow Shia rulers in neighbouring Iran, arch rival of the US.