A swathe of central Baghdad off limits to most Iraqis, the Green Zone — also known as the International Zone, or IZ — has been synonymous with the US military occupation of Iraq since the 2003 invasion that toppled Saddam Hussein.

Responsibility for the area on the banks of the Tigris, sealed off by grey concrete blast walls, was handed over to Iraq on Jan. 1, 2009 when US forces came under an Iraqi mandate, but six checkpoints controlling access were still jointly run.

"For more than the last year, we've just been in an oversight capacity at the ECPs (Entry Control Points)," Lt. Col. Greg McVay told Reuters. "At this point, they've got it and our soldiers are coming off." The handover comes as Washington prepares to end all combat operations in August and cut troop numbers from just under 90,000 now to 50,000 by Sept. 1.

US troops pulled out of Iraqi cities in June last year and should leave Iraq completely by the end of 2011 under a bilateral security pact.

The United States says it will press ahead with the withdrawal despite tensions following an inconclusive March 7 parliamentary election that has yet to yield a government. Iraq's Supreme Court certified the election results on Tuesday.

It was from within the Green Zone and the lavish Republican Palace that US officials began running Iraq in 2003. An estimated 100,000 civilians have been killed since the invasion.

Official figures released on Tuesday showed that a spike in civilian casualties in April had continued in May, with 275 killed as insurgents exploit post-election tensions.

A cross-sectarian alliance led by secular former Prime Minister Iyad Allawi came first in the election with the heavy backing of minority Sunni voters, but Iraq's two main Shiite blocs are bidding to form a governing majority.