The United States vowed support Wednesday to Iraqi leaders as they combat a militant offensive that has seized a large swathe of northern and north-central Iraq.

White House spokesman Jay Carney said Washington "strongly condemns the recent attacks in Iraq by the Islamic State of Iraq and the Levant."

"The United States will stand with Iraqi leaders across the political spectrum as they forge the national unity necessary to succeed in the fight against ISIL," he added in a statement.

Following the emboldened militants' advances closer to Baghdad, Washington also promised to boost aid to Iraq and is weighing drone strikes at the government's request.

"We will work with Congress to support the new Counterterrorism Partnerships Fund, which will provide flexibility and resources to help Iraq respond to emerging needs as the terrorist threat from ISIL continues to evolve," Carney said.

"Under the Strategic Framework Agreement, we will also continue to provide, and as required increase, assistance to the Government of Iraq to help build Iraq's capacity to effectively and sustainably stop ISIL's efforts to wreak havoc in Iraq and the region."

The request for air strikes has been turned down in the past, but Washington is now weighing several possibilities for more military assistance to Baghdad, including drone strikes, a Western official told AFP on condition of anonymity.

Resorting to such aircraft -- which remain highly controversial in Afghanistan and Pakistan -- would mark a dramatic shift in the US engagement in Iraq, after the last American troops pulled out in late 2011.

But State Department spokeswoman Jen Psaki stressed there were no current plans to send US troops back to Iraq, where around 4,500 Americans died in the eight-year conflict.

She also denied the offensive, in which the militants seized northern Mosul and then Tikrit, had caught Washington by surprise or marked a failure of US policy in the country it invaded in 2003.