BBC News with Marian Marshall.
President Obama has said America's commitment to the security of the Asia-Pacific region is "unshakeable".
Speaking in Tokyo, at the start of an Asian tour, Mr Obama said the United States had a direct stake in the future of the region because of its importance to the American economy.
He said the United States would continue to use its nuclear deterrent to protect its allies.
"Now let me be clear, so long as these weapons exist, the United States will maintain a strong and effective nuclear deterrent that guarantees the defence of our allies including South Korea and Japan.
But we must recognise that an escalating nuclear arms race in this region would undermine decades of growth and prosperity."
Human rights groups in the United States have welcomed President Barack Obama's decision to move five suspected al-Qaida members from the Guantanamo prison camp to stand trial in a civilian court in New York.
However, there has also been much criticism.