Outgoing U.S. Secretary of Defense Donald Rumsfeld has made a surprise visit to Iraq to thank U.S. troops' services there, a Pentagon spokesman said on Saturday.

"The trip is taking place," said Defense Department spokesman Lt. Col Todd Vician.

The purpose of Rumsfeld's visit "was to express his appreciation for what the troops are doing and to thank them and their families for the sacrifices they make on behalf of all Americans," he said.

Rumsfeld resigned in November, the day after President George W. Bush's Republicans lost control of both Houses of the Congress. He is to be replaced by Robert Gates, a former head of the Central Intelligence Agency (CIA).

The outgoing defense chief, who is under sharp criticism over his handling of the Iraq war and the treatment of detainees, defended the U.S. presence in Iraq on Friday in farewell remarks to Pentagon staff.

Rumsfeld said an early U.S. withdrawal from Iraq would be a "terrible mistake," and urged Americans to be patient with the Iraq war, saying there would be "dire consequences were we to fail here."

Source: Xinhua