President Barack Obama has confronted the paradox between war and peace, while receiving the Nobel Peace Prize in Oslo.

He accepted the Nobel Peace Prize on Thursday but acknowledged that as a wartime president he was a controversial choice.

Obama said in his acceptance speech that war is sometimes needed to pursue security and peace and that violent conflict will not end in our lifetime.

He said he was obligated to protect and defend the United States and that the use of force was sometimes not only necessary, but morally justified.

I face the world as it is and cannot stand idle in the face of threats to the American people, for, make no mistake, evil does exist in the world, Obama said.

A non-violent movement could not have halted Hitler's armies. Negotiations cannot convince Al Qaeda's leaders to lay down their arms, he continued.

Obama has ordered 30,000 U.S. troops to Afghanistan in a major expansion of the eight-year-old war just nine days before he accepted Nobel Peace Prize.

Some will kill and some will be killed, and so I come here with an acute sense of the costs of armed conflict, filled with difficult questions about the relationship between war and peace and our effort to replace one with the other, he said in the speech.

Whatever mistakes we have made, the plain fact is this, the United States of America has helped underwrite global security for more than six decades with the blood of our citizens and the strength of our arms.

Many Americans and even some Obama supporters abroad, appear to doubt that the president deserves a prize awarded in the past to Nelson Mandela, the Dalai Lama and Aung San Suu Kyi.

Obama also acknowledged criticism that he does not deserve the prize and has few tangible gains to show from his nearly 11 months in office, saying he was at the beginning, and not the end, of my labors on the world stage.