US President Barack Obama has accepted the 2009 Nobel Peace Prize with 'deep gratitude and 'humility' but warned that war was sometimes necessary despite its acute human tragedy.
'It is an award that speaks to our highest aspirations - that for all the cruelty and hardship of our world, we are not mere prisoners of fate,' Mr Obama said at the ceremony in Oslo.
'Our actions matter, and can bend history in the direction of justice,' he said at Oslo City Hall.
The US President admitted there had been criticism of his award, and accepted he did not belong in the same company as previous laureates like Martin Luther King and Nelson Mandela.
He also took on the irony that he was accepting a peace prize just after deciding to escalate the war in Afghanistan, sending 30,000 more US troops to fight in the region.
'The instruments of war do have a role to play in preserving the peace,' Mr Obama said.
'And yet this truth must coexist with another - that no matter how justified, war promises human tragedy.'
Mr Obama also paid tribute to anti-government demonstrators in Iran, Burma (officially Myanmar) and Zimbabwe.
A large security operation is under way in Oslo, where President Obama met Norway's Prime Minister Jens Stoltenberg and the Norwegian royal family before the ceremony.
Mr Obama is the third sitting US president, along with Theodore Roosevelt and Woodrow Wilson, to win the prize.
Jimmy Carter was honoured two decades after he left office.
Other prominent Nobel peace laureates include Nelson Mandela and Martin Luther King Jr.
John Hume and David Trimble were joint winners in 1998 for their work in securing The Good Friday Agreement in Northern Ireland.
