The United States' top military officer was in Baghdad Monday, an American army spokesman said, as US troops enter the final year of their mission in Iraq.

Admiral Mike Mullen's surprise visit comes with Iraqi Prime Minister Nuri al-Maliki in the midst of forming his cabinet ahead of a December 25 deadline, and with nationwide violence dramatically down from its peak in 2006 and 2007.

According to sources with knowledge of the cabinet negotiations, the interior minister will be an ally of Maliki while the defence minister will be from ex-premier Iyad Allawi's rival Iraqiya bloc.

Maliki was named premier-designate on November 25 and given 30 days to form a government. No new administration has formed in Iraq since the country held elections on March 7.

The chairman of the US Joint Chiefs of Staff's arrival in Baghdad was confirmed by Captain Dan Churchill, a US military press officer. Churchill did not give any further details of Mullen's visit.

Nearly 50,000 American troops remain stationed in Iraq, mostly focusing on training and advising Iraq's security forces. US soldiers do, however, participate in joint counter-terrorism operations, and can fire their weapons in self-defence.

Washington declared an official end to combat operations in Iraq at the end of August, and is due to withdraw completely from the country by 2012.