The U.S President Barack Obama has said the U.S 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 that 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. Yesterday's ceremony in the Norwegian capital came days after Mr Obama announced he was sending 30000 extra US soldiers to the war in Afghanistan.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.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.