US President Barack Obama and his Russian counterpart Dmitry Medvedev have signed a new nuclear arms reduction treaty that will cut their arsenals by about one third.
The successor to the 1991 Strategic Arms Reduction Treaty (START I) will limit operationally-deployed nuclear warheads to 1,550.
Mr Obama said the treaty had stopped the deterioration in Russian-US relations, and they were working together to pass strong sanctions on Iran.
"We are working together at the United Nations Security Council to pass strong sanctions on Iran and we will not tolerate actions that flout the NPT," he said, referring to the Nuclear Non-Proliferation Treaty.
However, disagreements remain with the US over its missile shield plans, but Mr Medvedev said he still hoped a compromise could be reached between Moscow and Washington.