A suicide car bomber killed at least 13 Iraqis at an entrance to Baghdad's Green Zone today, a year to the day since US forces captured Saddam Hussein.
Nineteen were wounded, four seriously, civilian hospital staff said.
A group led by Abu Musab al-Zarqawi has claimed responsibility for the bombing in a statement posted on the Internet.
"On this blessed day, a lion from the martyrs battalion struck a group of apostates and Americans in the Green Zone," Al Qaeda Organization for Holy War in Iraq said in a statement posted on a Web site used by Islamists.
It was not immediately possible to verify its authenticity.
The checkpoint, close to the main airport road, is on a main entrance to the sprawling complex which was Saddam's presidential palace. It now houses the US and other embassies and the offices of the US-backed Iraqi interim government.
Todays' explosion, at 9 a.m. (6 a.m. Irish time) when many Iraqis would be going to work in the Zone, shook buildings across Baghdad. Smoke poured from the site and helicopters circled overhead.
It was not clear if any Iraqi National Guards, who man the checkpoint, were hurt. Staff wheeled in a wounded man as doctors sewed up the bloodied hand of a young woman whose head was bandaged.
Sunni tribal leader Ghazi Yawar
Just north of Baghdad, at Tarmiya, witnesses said a car bomb went off on a main highway and US troops sealed the area.
Today is the first anniversary of the capture of Saddam by US forces after eight months on the run. At that time, US President George W. Bush and US military commanders hoped the former president's arrest would puncture guerrilla activity among his former supporters in the Sunni Arab minority.
However, violence has continued unabated and the death rate among US troops has risen since Saddam was dragged from a hideout dug in farmland near his home town of Tikrit.
Meanwhile, Iraq's figurehead interim president warned before national elections due on January 30th that violence and continued occupation could create a new Hitler.
"This could in the long term create an environment in which an Iraqi Hitler could emerge like the one created by the defeat of Germany and the humiliation of Germans in World War One," Sunni tribal leader Ghazi Yawar told the London-based Asharq al-Awsat newspaper.