U.S. President Barack Obama accepted the 2009 Nobel Peace Prize on Thursday, acknowledging the controversy over the choice of a wartime president and saying he reserved the right to take action to protect the United States.

In a speech at the award ceremony in Oslo, Obama said violent conflict would not be eradicated "in our lifetimes", there would be times when nations would need to fight just wars and he would not stand idle in the face of threats to the American people.

"Where force is necessary, we have a moral and strategic interest in binding ourselves to certain rules of conduct. And even as we confront a vicious adversary that abides by no rules, I believe that the United States of America must remain a standard bearer in the conduct of war," he declared.

Obama said that violence was especially justified when used on humanitarian grounds, adding that in the case of Al-Qaida, negotiations would not cause them to lay down their arms.

He called for tough action against countries that break international laws, such as sanctions that "exact a real price." Iran and North Korea, which are in nuclear stand-offs with the West, could not be allowed to "game the system," he said, referring to tactics employed by both countries in the past to draw out negotiations.

Obama said unity within the international community against "evil" forces was the only way to create a future with an alternative to violence, but said he did not see wars ending in our lifetimes.

"I believe that we must develop alternatives to violence that are tough enough to change behavior - for if we want a lasting peace, then the words of the international community must mean something," he said.

"Those regimes that break the rules must be held accountable. Sanctions must exact a real price. Intransigence must be met with increased pressure - and such pressure exists only when the world stands together as one."

Nevertheless, he said, "I, like any head of state - reserve the right to act unilaterally if necessary to defend my nation." Still, he said, "I am convinced that adhering to standards strengthens those who do, and isolates - and weakens - those who don't."

As he opened his address, Obama acknowledged the criticism that his accolades were undeserved.

"I receive this honor with deep gratitude and great humility," he said. "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. Our actions matter, and can bend history in the direction of justice."

"And yet I would be remiss if I did not acknowledge the considerable controversy that your generous decision has generated," said Obama. "In part, this is because I am at the beginning, and not the end, of my labors on the world stage.

"Compared to some of the giants of history who have received this prize - Schweitzer and King; Marshall and Mandela - my accomplishments are slight," he said. And then there are the men and women around the world who have been jailed and beaten in the pursuit of justice; those who toil in humanitarian organizations to relieve suffering; the unrecognized millions whose quiet acts of courage and compassion inspire even the most hardened of cynics.

"I cannot argue with those who find these men and women - some known, some obscure to all but those they help - to be far more deserving of this honor than I," he said.

Obama said during that he did not "bring with me today a definitive solution to the problems of war."

"What I do know is that meeting these challenges will require the same vision, hard work, and persistence of those men and women who acted so boldly decades ago," he said. "And it will require us to think in new ways about the notions of just war and the imperatives of a just peace."

"We must begin by acknowledging the hard truth that we will not eradicate violent conflict in our lifetimes," Obama added. "There will be times when nations - acting individually or in concert - will find the use of force not only necessary but morally justified."

While expressing belief in Martin Luther King, Jr.'s statement that "Violence never brings permanent peace," Obama also said that he had to 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," he said. "A non-violent movement could not have halted Hitler's armies. Negotiations cannot convince Al-Qaida's leaders to lay down their arms. To say that force is sometimes necessary is not a call to cynicism - it is a recognition of history; the imperfections of man and the limits of reason."

"So let us reach for the world that ought to be ? that spark of the divine that still stirs within each of our souls," Obama said, as he brought his address to an end.

"We can acknowledge that oppression will always be with us, and still strive for justice. We can admit the intractability of depravation, and still strive for dignity. We can understand that there will be war, and still strive for peace.

"We can do that - for that is the story of human progress; that is the hope of all the world; and at this moment of challenge, that must be our work here on Earth," he concluded.

Obama: If I fail, no prize will hide it

Hours before he was set to receive the prize, Obama said that he hoped to achieve his goal of advancing U.S. interests and peace around the world.

"If I am successful in those tasks, then hopefully some of the criticism will subside, but that is not really my concern. If I am not successful, than all the praise and awards in the world will not disguise that."

Addressing criticism that the award was premature, Obama told reporters after landing in Oslo that he does not doubt there are others who may be more deserving of the honor.

"I have no doubt that there are others that may be more deserving. My task here is to continue on the path that I believe is not only important for America but important for lasting peace in the world," Obama said in response to a journalist's question on how he planned to use the accolade to advance his goals.

Obama said he wanted to continue working on issues that are important for the U.S. and for building lasting peace and security in the world, such as halting the spread of nuclear weapons, addressing climate change and stabilizing Afghanistan.

Obama arrived in Oslo on Thursday morning and met Norway's Prime Minister Jens Stoltenberg and the Norwegian royal family before receiving his medal.

As he and his entourage arrived for the ceremony, a crowd gathered and held up a yellow banner, which read: "Obama You Won It, Now Earn It."

Obama is the third sitting U.S. president, along with Theodore Roosevelt and Woodrow Wilson, to win the Nobel Peace Prize. Jimmy Carter was honoured two decades after he left office. Other prominent Nobel peace laureates include Nelson Mandela, Martin Luther King Jr. and Mother Teresa.

The ceremony comes just nine days after Obama ordered 30,000 more U.S. troops to Afghanistan to break the momentum of the Taliban. The troop announcement, so shortly before the Nobel ceremony, was an "interesting coincidence of history" not lost on the president, a senior administration official said.

He is due to receive the award at a ceremony in Oslo City Hall starting after flying overnight from Washington. Aides said Obama, known for his soaring rhetoric, was still working on the estimated 20- to 25-minute
speech in the hours before his journey.

Some polls show that while many Americans are proud Obama is receiving the award, a majority feel it is undeserved. Americans remain anxious about the economy, nudging Obama's approval ratings down to 50 percent or below and potentially hurting his Democratic Party in congressional elections next year.

Many people were stunned, including some in the White House, when the Nobel committee announced in October it was awarding the peace prize to Obama for "his extraordinary efforts to strengthen international diplomacy and cooperation between peoples" and cited his push for nuclear disarmament.

Critics called the decision premature, given that Obama, who took office in January, had achieved few tangible gains as he grapples with challenges ranging from the war in Afghanistan and nuclear stand-offs with Iran and North Korea to climate change.

Obama is due to join scores of other world leaders in Copenhagen next week at the climax of a UN conference on climate change, though legislation to curb greenhouse gas emissions is stuck in the U.S. Congress.

Eco-activists in Oslo intend to keep pressure on Obama during the Nobel celebrations to sign a deal in Copenhagen.

The administration official said Obama would not shy away from the Afghanistan war in his speech in Oslo and would address the apparent contradiction of a president whose country is involved in two wars receiving a peace prize.

"Right now, he has a range of foreign policy and national security initiatives, all of which are designed toward achieving greater peace and security in the world," the official said.

"He feels in many ways that he has not fully earned the award yet. He is at the beginning of his presidency and in many ways at the beginning of his work on behalf of peace."

Obama has been widely credited with improving America's global image after the eight-year presidency of George W. Bush, who alienated allies with his mostly unilateral policies, like the 2003 U.S.-led invasion of Iraq.