The last batch of U.S. combat forces in Iraq crossed into neighboring Kuwait on Sunday morning, the U.S. military has told media.

A convoy of about 100 armored vehicles carrying about 500 U.S. troops crossed the border at around 7:30 local time (0430 GMT) before heading for military camps in Kuwait, a transit point for the U.S. mission in Iraq.

The exit marked the completion of the U.S. military withdrawal from the Middle East country after a nine-year war that cost nearly 4,500 American and more than 100,000 Iraqi lives.

The pullout was completed ahead of the Dec. 31 deadline set by U.S. President Barack Obama, who promised during his presidential election campaign to bring U.S. soldiers back home before 2012.

U.S. Defence Secretary Leon Panetta attended a flag-lowering ceremony at a U.S. military base west of the Iraqi capital of Baghdad on Thursday to mark the end of the military involvement.

Although Panetta said the war set Iraq on a path to democracy, controversy still remains over the reasons given to invade Iraq and whether Iraqi security forces can defend the nation after the U.S. departure.

Accusing the then Iraqi government under Saddam Hussein of harboring weapons of mass destruction, the United States invaded Iraq in 2003. No evidence has been found to support the claim.

Saddam was captured in December 2003 and hanged three years later.