The United States must uphold moral standards when waging wars that are necessary and justified, President Barack Obama said on Thursday as he accepted the Nobel Prize for Peace.

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.

Nine days after ordering 30,000 more U.S. troops to Afghanistan to break the momentum of the Taliban, Obama acknowledged the criticism of those who have said it was wrong and premature to award the Nobel accolade to a president still in his first year in office and escalating a major war.

He said America's adherence to moral standards, even in war, was what made it different from its enemies.

That is a source of our strength. That is why I prohibited torture. That is why I ordered the prison at Guantanamo Bay closed. And that is why I have reaffirmed America's commitment to abide by the Geneva Conventions, he said.

By pledging to close the Guantanamo camp for foreign terrorist suspects on Cuba, and moving to bring inmates to trial on U.S. soil, Obama has attempted to recover the moral high ground that critics of the United States accused his predecessor George W. Bush of surrendering by waging a no-holds-barred war on terror.

We lose ourselves when we compromise the very ideals that we fight to defend. And we honor those ideals by upholding them not just when it is easy, but when it is hard, Obama said.

SANCTIONS MUST BITE

Acknowledging a reflexive suspicion of America, the world's sole military superpower, he said his country could not act alone in confronting global challenges in Afghanistan, Somalia or other troubled regions.

In seeking alternatives to force, it was necessary to be tough.

Those regimes that break the rules must be held accountable. Sanctions must enact a real price, Obama said in a passage that addressed North Korea's nuclear arsenal and U.S. suspicions that Iran, too, seeks to acquire the bomb.

It is...incumbent upon all of us to insist that nations like Iran and North Korea do not game the system, Obama said. Those who seek peace cannot stand idly by as nations arm themselves for nuclear war.

At a news conference earlier, Obama reaffirmed that U.S. troops would begin transferring responsibility for Afghan security to local forces in July 2011 but said there would be no precipitous drawdown.

Acknowledging the controversy surrounding his prize, he said: 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.

He said that meant pursuing a world free of nuclear weapons and countering proliferation; addressing climate change; stabilizing countries like Afghanistan; mobilizing an international effort to deal with terrorism that is consistent with our values and ideals; and addressing development issues.

Some of these initiatives were beginning to bear fruit, Obama said.

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.

Norwegian Prime Minister Jens Stoltenberg told journalists the prize was well deserved and can contribute in itself to strengthening the efforts of the president to work for peace.

On a rainy day with temperatures just above freezing, thousands lined heavily guarded Oslo streets to greet Obama.

Only handfuls of protesters were visible, with one group holding a sign reading: Obama you won it, now earn it.

Environmentalists in the crowd called on the U.S. leader to sign an ambitious deal to fight global warming when he visits nearby Copenhagen next week for the climax of a U.N. climate conference involving nearly 200 countries.

Cuba Afghanistan George W. Bush Somalia