They criticised Obama's decision to withdraw all US troops in December 2011, instead of leaving behind a residual force, saying the militants' advances were "as tragic as they were predictable".
Obama has long said that ending the deeply unpopular Iraq war was one of his key foreign policy successes and US public opinion overwhelmingly backed the withdrawal.
But images of al-Qaeda's black flag flying in Fallujah, where more than 1300 US troops were killed during the war, threatened to undermine the White House narrative of success.
John Kerry, the US Secretary of State, said there was no chance of US ground troops returning to Iraq but did not rule out using American drones to support the Iraqi Government's counter-offensive.
"We're not contemplating putting boots on the ground," Kerry said during a visit to Jerusalem. "This is their fight, but we're going to help them in their fight."
Nearly 9000 people were killed in Iraq last year, making it the bloodiest year of sectarian violence since 2008. Alarmed at the bloodshed, the US quietly began to step up its supply of Hellfire missiles to Iraq's fledgling air force and will deliver basic reconnaissance drones later this year.
But the increased military aid, combined with CIA intelligence support, has not been enough to see off a resurgent al-Qaeda in western Iraq.
Their fighters now control large swathes of territory in Iraq and Syria, operating as the Islamic State of Iraq and the Levant (ISIL) or the Islamic State of Iraq and Syria (ISIS).
But Iraq military sources appeared confident of retaking control.
The takeover of Fallujah and parts of Anbar provincial capital Ramadi, further west, is the first time militants have exercised such open control in major cities since the height of the bloody insurgency that followed the US-led invasion of 2003. A spokesman said yesterday it was preparing for a major offensive aimed at retaking Fallujah and driving ISIS forces out of nearby Ramadi.
Lieutenant General Rasheed Fleih, the head of the Anbar military command, said his troops and Sunni tribes still loyal to the Shia-led Government in Baghdad would take back the cities within "two to three days".
Military aircraft struck Ramadi yesterday and government officials claimed to have killed 25 militants. Meanwhile, a series of car bombs in Baghdad killed at least 20 people.
US forces were completely withdrawn from Iraq after Washington and Baghdad failed to reached an agreement on the legal status of American troops.
Fighting in Anbar
Latest Iraqi, al-Qaeda clashes
34 dead from fighting in Anbar province, say Iraqi officials
22 soldiers and 12 civilians dead, as well as an unknown number of militants around Ramadi
58 people have been wounded in the combat
The Iraq conflict
9000 Iraqis killed last year
1300 US troops killed in Fallujah during the Iraq War
2 major assaults on Fallujah after the 2003 invasion
US troops and local tribes finally beat al-Qaeda back in 2006-07
Al-Qaeda's Islamic State of Iraq and the Levant (Syria)
* Commonly known as ISIS or ISIL
* Militants and supporters use black al-Qaeda flags
* The militants have been tightening their grip on Anbar for months
* They are trying to create a state ruled according to strict medieval Sunni Islamic practice across the Iraqi-Syrian border.
