US President-elect Barack Obama has announced former rival Hillary Clinton as his choice for secretary of state.

He also says Robert Gates will remain as defense secretary as the incoming administration seeks to wind down the US role in Iraq and Afghanistan.

Obama announced his major officials for foreign affairs and national security on Monday, a week after his economic team.

Obama first introduced Clinton.

Barack Obama, US President-elect, said, "I have known Hillary Clinton as a friend, a colleague, a source of counsel, and a tough campaign opponent. She possesses an extraordinary intelligence and a remarkable work ethic. I am proud that she will be our next Secretary of State."

Clinton says the US must rely on its friends to help confront threats like global warming and terrorism -- a rebuke to the go-it-alone approach of President George W. Bush.

Gates' presence in Chicago made him a visible symbol of the transition in power.

Obama says he will give the military a new mission as soon as he takes office. He calls it "responsibly ending the war in Iraq through a successful transition to Iraqi control."