The US military formally ends combat operations in Iraq on Tuesday as President Barack Obama seeks to fulfil a promise to end the war despite persistent instability and attacks that kill dozens at a time.

A US Soldier takes down the vehicle numbers of humvees loaded up on a trailer as they prepare to leave Iraq at Balad Base 80 km (50 miles) north of Baghdad August 27, 2010. [Agencies]

US troop numbers were cut to 50,000 in advance of the August 31 milestone in the 7-1/2-year-old war launched by Obama's predecessor, George W. Bush, whose stated aim was to destroy Iraqi weapons of mass destruction. No such weapons were found. 

The six remaining US military brigades will turn their focus to training and advising Iraqi police and troops as Iraq takes on responsibility for its own destiny ahead of a full withdrawal of US forces by the end of next year.

"The story is not about 50,000. The story is that we are continuing to be committed to Iraq, but our commitment is going to change," the outgoing US military commander in Iraq, General Raymond Odierno, said last week.

"It's no longer one that is focused solely on a military commitment. We are building a different relationship with Iraq, one that's focused on economic development, one that's focused on technological development, one that's focused on political development and cultural development."