U.S. Secretary of Defense Robert Gates flew into Iraq in a surprise visit on Wednesday, a day after the U.S. military formally ended combat mission in the country, an Iraqi official television said.

Gates is expected to attend a change of command ceremony for the U.S. military outgoing commander in Iraq, General Raymond Odierno, who is due to hand over to the incoming commander, Lieutenant General Lloyd Austin, the state-run television of Iraqia said.

U.S. Vice President Joe Biden is also in Iraq since Monday for talks with Iraqi top officials to urge them to form a government nearly six months after the parliamentary elections in the country.

Both U.S. leaders are due to attend a ceremony marking the end of the U.S. combat mission and the start of the new "advise and assist" mission, dubbed "Operation New Dawn."

Gates' visit came hours after U.S. President Barack Obama announced the end of U.S. combat mission in Iraq.

U.S. troops in Iraq had been reduced to below 50,000 soldiers. Washington said that the remaining U.S. troops will remain in Iraq to conduct support and training missions.