Top US officials turned on Iraq's leader, blaming his "sectarian" policies for the country's crisis as Washington Thursday weighed calls for air strikes on Sunni militants bearing down on Baghdad.

The sharp criticism of Prime Minister Nuri al-Maliki came as he scrambled to repel an insurgent onslaught that has seen an entire province and parts of three others fall out of government control in an offensive that could threaten Iraq's very existence.

The militants' swift advance has sparked international alarm and the United Nations has warned that the crisis was "life-threatening for Iraq".

Hundreds of thousands of Iraqis have been displaced in the nine days of fighting and an unknown number killed, while dozens of Indians and Turks have been kidnapped.

With President Barack Obama mulling a request by Baghdad for air strikes on the advancing militants, US officials castigated Maliki, publicly echoing long-held criticisms among his domestic opponents of sectarianism.

US Vice President Joe Biden urged greater political inclusion in Iraq in phone calls with Maliki and other Iraqi officials, the White House said.

Biden "stressed the need for national unity in responding to the ISIL threat against all Iraqi communities," in calls with Maliki, Iraqi Council of Representatives Speaker Osama al-Nujaifi, and President of the Iraqi Kurdistan Region Masoud Barzani, it said.

The top-ranking military officer, General Martin Dempsey, and former US commander David Petraeus both also rounded on the premier.

"There is very little that could have been done to overcome the degree to which the government of Iraq had failed its people," Dempsey said.

Petraeus warned at a conference in London that Washington risked becoming an "air force for Shiite militias" and supporting "one side of what could be a sectarian civil war" if political reconciliation were not agreed.

- Iraq requests strikes -

The remarks came after Iraqi Foreign Minister Hoshyar Zebari told reporters in Saudi Arabia that Baghdad had asked Washington "to conduct air strikes against terrorist groups".

Zebari acknowledged "the need for drastic political solutions."

Washington has deployed an aircraft carrier to the Gulf and sent military personnel to bolster security at its Baghdad embassy, but Obama insists a return to combat in Iraq is not in the cards.

The United States spent billions of dollars over several years training and arming Iraqi security forces after disbanding the Sunni-led army following the 2003 invasion that ousted dictator Saddam Hussein.

But the security forces wilted when faced with the militant offensive on June 9 which saw insurgents quickly capture Mosul, a city of some two million people, and then parts of Salaheddin, Kirkuk and Diyala provinces.

Some abandoned their vehicles and uniforms when faced with the insurgents, which are led by ISIL fighters but also include Saddam loyalists.

The Sunni fighters have been led by the powerful Islamic State of Iraq and the Levant, but also include a wide coalition of other Sunni Arab militant groups, as well as loyalists of executed dictator Saddam Hussein.

Though the alliance has made significant territorial gains, the wildly divergent ideologies of its constituent groups means it may struggle to survive over time, analysts say.

And while they struggled in the early part of the offensive, Iraq's security forces appear to be performing better in recent days, managing to make advances in certain areas, though militants have made their own gains elsewhere.

- 'Stiffening their resistance' -

The Pentagon has noted that Iraqi forces were "stiffening their resistance" around Baghdad, while the increasingly open assistance from Shiite militia groups towards government soldiers and policemen has also played a major role.

In a bid to reverse insurgent gains, Maliki fired disgraced security commanders and stood alongside fierce rivals in a rare show of political unity.

"We will teach (militants) a lesson and strike them," the premier said, insisting that while security forces had suffered a "setback", they had not been defeated.

With regional tensions rising, Iranian President Hassan Rouhani said the Islamic republic "will do everything" to protect Shiite shrines in Iraqi cities against the militant assault.

And Saudi Arabia warned of the risks of a civil war in Iraq with unpredictable consequences for the region, while the United Arab Emirates recalled its envoy to Baghdad, voicing concern over "exclusionary and sectarian policies".

An assault on the Baiji oil refinery in Salaheddin province, north of Baghdad on Wednesday further spooked international oil markets, though officials and analysts said the country's vast oil exports were safe -- for now.

Officials said security forces controlled the refinery, but that fighting was continuing sporadically on Thursday.

World oil producers have cautiously watched the unfolding chaos in Iraq, which exports around 2.5 million barrels of oil per day, and said that the country's vast crude supplies, mostly in the south, were safe for now.