WASHINGTON — President Obama was awarded the Nobel Peace Prize on Friday for his “extraordinary efforts to strengthen international diplomacy and cooperation between peoples,” a surprising honor that came less than nine months after he made United States history by becoming the country’s first African-American president.

The award, announced in Oslo by the Nobel Committee while much of official Washington — including the president — was still asleep, cited in particular the president’s efforts to rid the world of nuclear weapons.

“He has created a new international climate,” the committee said.

For Mr. Obama, at 48 one of the nation’s youngest presidents, the award is an extraordinary recognition that puts him in the company of world leaders such as Mikhail S. Gorbachev, who won for helping to bring an end to the cold war, and Nelson Mandela, who sought an end to apartheid. But it is also a potential political liability at home; already, Republicans are criticizing the president, contending he won more for his “star power” than his actual achievements.

Appearing in the Rose Garden, Mr. Obama said he was “surprised and deeply humbled” by the committee’s decision, and quickly put to rest any speculation that he might not accept the honor. Describing the award as an “affirmation of American leadership on behalf of aspirations held by people in all nations,” he said he would accept it as “a call to action.”

“To be honest,” the president said “I do not feel that I deserve to be in the company of so many of the transformative figures who have been honored by this prize, men and women who’ve inspired me and inspired the entire world through their courageous pursuit of peace.”

The news shocked people in Oslo — where an audible gasp escaped the audience when the decision was announced — and in Washington, where top advisers to Mr. Obama said they had no idea it was coming. The president was awakened shortly before 6 a.m. by his press secretary, Robert Gibbs, who delivered the news.