United States President Barack Obama has received the Nobel peace prize at a ceremony in Norway.
Mr Obama was awarded the prize in October for his "extraordinary efforts to strengthen international diplomacy and co-operation between peoples".
In his acceptance speech in Oslo on Thursday, Mr Obama spoke of his gratitude and humility - but said others might be more deserving, the BBC reports.
The ceremony came days after Mr Obama announced he was sending 30,000 extra American soldiers to the war in Afghanistan.
Mr Obama said in his speech that the US must uphold moral standards when waging wars that are necessary and justified.
The president defended his country's role in Afghanistan, arguing the use of force could bring lasting peace. He said a non-violent movement could not have halted Hitler's armies, and negotiations will not convince the al-Qaeda's leaders to lay down their arms.
Anti-war demonstrators gathered outside the city hall in Oslo, where the ceremony took place.
There was a mixed reaction when Mr Obama was named as the winner of this year's prize, becoming the fourth US president to be given the honour.
Mr Obama's elevation to the rank of fellow laureates such as Nelson Mandela, Mother Teresa and Martin Luther King, before he has even spent a year in office, has sparked fierce debate.
Critics also said it was inappropriate for the honour to go to the commander-in-chief of a country involved in conflicts in Iraq and Afghanistan.
Acknowledging the controversy, Mr Obama said he accepted the award with humility, adding: "Compared to some of the giants of history who have received this prize... my accomplishments are slight."
He could not argue with those who said many previous laureates were "far more deserving" of the honour than him, he said.
Nobel prizes for chemistry, medicine, physics, literature and economics will be presented later in the Swedish capital, Stockholm.