President Obama receives his Nobel Peace Prize medal - Copyright Nobel Media AB 2009/NRK
President Barack Obama has said the US must uphold moral standards when waging wars that are necessary and justified, as he accepted his Nobel Peace Prize.
In his speech in Oslo, he defended the US role in Afghanistan, arguing the use of force could bring lasting peace.
He also said his accomplishments were slight compared with other laureates.
Mr Obama was given the prize in October for his "extraordinary efforts to strengthen international diplomacy and co-operation between peoples".
Thursday's ceremony in the Norwegian capital came days after Mr Obama announced he was sending 30,000 extra US soldiers to the war in Afghanistan.
Paul Reynolds, BBC world affairs correspondent
He is careful to say that America respects the cultures and traditions of others - again, trying to project the US as a defender not an aggressor.
But he also wants to stand up for "universal values". It is a difficult balancing act.
His statement that "America has never fought a war against a democracy" might raise the issue as to whether it has fought on the side of non-democracies.
There was a mixed reaction when he 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.
Defending his Afghan troop deployment, Mr Obama said there were times when "the use of force [was] not only necessary but morally justified," as long as force was proportionate and civilian casualties minimised.
"Instruments of war do have a role to play in preserving the peace", he said.
"A non-violent movement could not have halted Hitler's armies," he added. "Negotiations cannot convince al-Qaeda's leaders to lay down their arms."
He said the US "must remain a standard bearer in the conduct of war" to differentiate it from "a vicious adversary that abides by no rules".
Mr Obama also emphasised alternatives to violence, stressing the importance of diplomacy and sanctions to confront nations like Iran and North Korea over their nuclear programmes.
While Russia and America were working to reduce their nuclear stockpiles, he said the international community must ensure Tehran and Pyongyang did not "game the system".
"Let us reach for the world that ought to be," Mr Obama said. "We can understand that there will be war, and still strive for peace."
He paid tribute to anti-government demonstrators in Iran, Burma and Zimbabwe, and said the US would always stand on the side of those who sought freedom.
Some anti-war demonstrators gathered outside city hall, where the ceremony was held.
"We are protesting against him because... we don't think he is a man of peace," one of them told AFP news agency.
Amid high security, the US president earlier signed the Nobel Prize book of previous laureates after arriving in Oslo with his wife, Michelle.
There has been some disappointment in Norway at Mr Obama's decision to stay only one day, even though Nobel ceremonies are usually held over three, and decline a traditional lunch with the king.
At a news conference with Norwegian Prime Minister Jens Stoltenberg, Mr Obama said he and his wife wished they could stay longer.
In the evening, Mr Obama appeared alongside Michelle on their hotel balcony and waved to a torchlight procession below. The couple were then due to go to the traditional winners' banquet.
The Nobel Prizes for chemistry, literature, medicine, physics and economics are also being presented, in the Swedish capital Stockholm.
Each laureate, including Mr Obama, receives a diploma, a medal and 10m kroner ($1.4m; £865,000).
Coinciding with the Nobel ceremony, a statue of Mr Obama as a young boy was unveiled in the Indonesian capital, Jakarta, at a park, near where the president lived between 1967 and 1971.