In PHOTOS: US troops leave Iraq after nine years

The United States on Thursday formally ended their nine-year mission in Iraq, which began with the invasion to oust Saddam Hussein, with a low-key ceremony in Baghdad.

Today, there are around 4,000 American soldiers in Iraq, but they will depart in the coming days.

US Defence Secretary Leon E Panetta marked the occasion with a speech in a fortified concrete courtyard at the Baghdad airport. "Let me be clear: Iraq will be tested in the days ahead -- by terrorism, and by those who would seek to divide, by economic and social issues, by the demands of democracy itself," Panetta said. "Challenges remain, but the U.S. will be there to stand by the Iraqi people as they navigate those challenges to build a stronger and more prosperous nation."

The withdrawal ends a war that left tens of thousands of Iraqis and nearly 4,500 American soldiers dead, many more wounded, and 1.75 million Iraqis displaced, after the 2003 US-led invasion unleashed brutal sectarian fighting.

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Members of the US military board an Air Force C-130 transport plane to leave Iraq at the Baghdad Diplomatic Support Center in Baghdad.

Members of the 1st cavalry 1st Brigade Special Troops Battalion fold the US flag during a handover ceremony of Camp Kalsu to the Iraqi army, south of Baghdad.

A US navy crew carries a folded flag as they prepare to hand over their base to Iraqi forces in Iraq's southern province of Basra.

Iraqi soldiers celebrate during a ceremony where Camp Echo is handed over to Iraqi security forces from the U.S. military forces in Diwaniya, 150 km south of the capital Baghdad.

Iraqi soldiers stand in line during a ceremonial handover of Camp Echo from the U.S. military forces to Iraqi security forces in Diwaniya.

Iraqi navy crew stand at attention during the handover ceremony at Iraq's southern province of Basra.

A New York state flag with signatures of U.S. Air Force airmen are seen on concrete blast protection walls at the former U.S. Sather Air Base near Baghdad December 14. The former base that is currently still operational was handed over to the Baghdad Diplomatic Support Center run by the State Department on December 1.

US Army Sgt Omar Sprott, from Brooklyn, New York, of the 115th Brigade Support Battalion carries his luggage in preparation for leaving Camp Kalsu near Hillla, Iraq. 

A member of the US Air Force waves while boarding a plane before flying to the U.S., at al-Asad air base in Iraq's western province of Anbar.

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