Al Qaeda's Iraqi affiliate said on Sunday it carried out a coordinated suicide bomb and gun attack on the country's justice ministry last week that killed at least 25 people in the centre of Baghdad.
The assault near the heavily fortified Green Zone, where several Western embassies and government offices are located, fanned fears about Iraq's still fragile security a decade after the invasion that ousted Saddam Hussein.
Three car bombs exploded and a suicide bomber blew himself up in broad daylight in the heart of the capital on Thursday.
Another suicide bomber then walked into the justice ministry and set off his device while militants attacked the building. Iraqi security forces eventually regained control.
Islamic State of Iraq (ISI), an umbrella group for al Qaeda-linked Sunni Muslim insurgents, said it had ordered the suicide bombers to attack the building floor by floor and "liquidate" its enemies inside.
Islamic State of Iraq accuses Iraq's Shi'ite Muslim-led government of oppressing Sunnis.
"In a blessed raid among a series of operations for revenge ... Baghdad's knights undermined another vicious bastion which was always a tool against Sunnis, torturing, terrifying, imprisoning and executing them," al-Qaeda said in a statement published online.
Iraq's power-sharing government has been all but paralysed since U.S. troops left more than a year ago. Prime Minister Nuri al-Maliki, a Shi'ite, is facing protests in the country's Sunni heartland, which shares a porous border with Syria.
Violence has intensified as Sunni opposition protests have swelled and Iraq's al Qaeda affiliate has urged the protesters to take up arms against the government.
Security experts say al Qaeda-linked militants have been regrouping in the western province of Anbar and crossing into Syria to fight alongside mainly Sunni rebels battling forces loyal to Syria's President Bashar al-Assad, who belongs to an offshoot of Shi-ite Islam.