Iraq’s al-Qaeda affiliate claimed responsibility Tuesday for a jailbreak from the infamous Abu Ghraib prison that unleashed hundreds of militants into an already unstable region and boosted the group’s resurgent fortunes in Iraq and Syria.

The Islamic State of Iraq and the Levant said in a statement that hundreds of prisoners were freed late Sunday in two coordinated assaults in which fighters used suicide bombs and mortars to storm the two top security prisons on Baghdad’s outskirts at Abu Ghraib and Taji. Both were once run by the U.S. military and housed the country’s most senior al-Qaeda detainees.

At least 26 members of the Iraqi security forces and more than a dozen prisoners were killed as the insurgents battled to breach the prison walls, Iraqi officials said. At Abu Ghraib, the attackers were apparently successful, and Iraq’s Interior Ministry said all the escapees came from that facility. The assailants were helped at both prisons, the ministry said, by “collusion” from guards inside.

The Iraqi government has not confirmed how many prisoners were set free, and officials said they were still trying to count those who got away. In Washington, U.S. officials put the number of escapees at between 500 and 600, including a significant number of al-Qaeda operatives.

The Islamic State of Iraq and the Levant said that the inmates freed in the raid included “over 500 jihadis ... who were trained by war and prepared by battle.” They have been “taken to secure locations away from the enemy,” the statement added.

The Iraqi security forces were still hunting for fugitives Tuesday, and at least some were recaptured in the dragnet, the government said. Reports in the Iraqi media said the families of security force members and those who fought al-Qaeda alongside U.S. troops were fleeing areas in western Iraq, fearing revenge attacks from escaped militants.

The scale of the attacks against the heavily guarded facilities reinforced an impression building among many Iraqis that their security forces are struggling to cope with a resurgent al-Qaeda since U.S. forces withdrew in 2011, taking with them much of the expertise and technology that had been used to hold extremists at bay.

The jailbreak coincided with a relentless wave of bombings blamed on al-Qaeda. The bombings have claimed the lives of hundreds of civilians in recent months, bringing back levels of violence not seen since U.S. troops surged into Iraq in 2007 in a bid to reverse the bloodshed and to assert Iraqi government control.

The gains of the surge are now in jeopardy, said Aymenn al-Tamimi, a fellow with the Philadelphia-based Middle East Forum who monitors extremist activity in Iraq and Syria.

“This is a significant milestone in the resurgence of al-Qaeda in Iraq,” he said. “A good deal of the progress achieved from 2006 onwards has essentially been undone now.”

The operation will also help accelerate the group’s ascendancy in Syria, where it has been rapidly expanding at the expense of more-moderate rebel groups, said Charles Lister of the London-based IHS Jane’s Terrorism and Insurgency Center.

“There’s no underestimating the boost to morale,” he said. “The fact that the Islamic State has managed to secure territory of its own in northern Syria all adds to a gradual trend of increasing confidence and strength of al-Qaeda in Iraq and in Syria.”

Unless they are recaptured, those who have escaped will add to the pool of experienced operatives sustaining al-Qaeda’s growing influence in both countries, he said. Extremist Web sites claimed that the escapees include a number of foreign fighters captured by the U.S. military in 2006 and 2007. They could seek to join the war in Syria against the government of President Bashar al-Assad at a time when foreign fighters are flocking there from across the region.

Al-Qaeda’s Iraq affiliate has undergone a number of identity transformations since it first announced its existence to counter the U.S. occupation in 2004. In 2006, it rebranded itself as the Islamic State of Iraq, then in April announced that it would henceforth use the name Islamic State of Iraq and the Levant, thereby extending the reach of its activities into Syria and absorbing another al-Qaeda affiliate, the Syrian group Jabhat al-Nusra.

A State Department spokesman declined to comment Monday, other than to say that Iraqi officials are “in pursuit of the escaped prisoners to return them to captivity.”

Abu Ghraib prison came under attack multiple times while under U.S. control. In 2005, more than 60 insurgents assaulted the facility with car bombs, rockets and grenades in a failed attempt to breach the walls and trigger a mass escape. Al-Qaeda in Iraq claimed responsibility for the attack, which injured 36 people inside the prison and left 50 attackers dead.

Sly reported from Beirut. Ahmed Ramadan in Beirut and Joby Warrick in Washington contributed to this report.