During a farewell tribute ceremony at the Pentagon, Obama praised Gates for his services for the country, and awarded him the Presidential Medal of Freedom -- the highest award a president can confer to a civilian.

"I can think of no better way to express my appreciation to someone I have come to admire and whom I consider a friend," Obama said. "I can think of no better way to express the gratitude of the nation for Bob Gates than with a very special recognition."

Calling the award a big surprise, Gates said he was deeply honored and moved by the president's presentation of the award. He thanked President George W. Bush for nominating him for the job in 2006, and President Obama for retaining him in it during the change in administrations in 2009.

Gates announced last August that he would quit the job at the middle of this year. His successor -- Director of the Central Intelligence Agency Leon Panetta -- will take over the job starting next month.