The United States and Russia signed a landmark strategic nuclear disarmament treaty on Thursday.

Under the new Strategic Arms Reduction Treaty (START), the former Cold War foes will be allowed a maximum of 1,550 deployed warheads, about 30 percent lower than a limit set in 2002.

It also imposes limits on the intercontinental ballistic missiles needed to deliver the warheads.

Agreed after months of hard bargaining, the two presidents signed the treaty in the renaissance Spanish Hall of Prague Castle.

Following private talks, Medvedev and Obama sat side by side to sign the treaty which was also a major step in their efforts to "reset" often strained relations.

Obama called it "an extraordinary event" and an "important milestone" for attempts to restrict nuclear arms as well as US-Russia relations.

He called the treaty "one step on a longer journey ... this treaty will set the stage for further cuts."

Obama said the two nations, which between them have more than 95 percent of the world's nuclear arms, had to show "responsible global leadership."

Medvedev said the treaty "enables us to rise to a higher level of cooperation between Russia and the United States" while also declaring that the negotiations "have not been simple".

It was in the Czech capital exactly 12 months ago that Obama gave a keynote speech committing the United States to the aim of a world without nuclear weapons.

He reaffirmed his comment from a year ago that disarmament was "a long term goal, one that may not be achieved in my lifetime."

US shifts doctrine

Obama this week announced a shift in U.S. nuclear doctrine, pledging never to use atomic weapons against non-nuclear states, as he sought to build momentum for an April 12-13 nuclear security summit in Washington.

The U.S. president reaffirmed the long-term goal he set in a speech at the same Prague Castle a year ago to work towards a world without nuclear weapons and said Medvedev would visit the United States later this year to discuss further cooperation, including withdrawing short-range tactical nuclear weapons.

Both men said they hoped and expected the new treaty would be ratified this year. Medvedev, mindful of past problems in winning approval in the treaty-shy U.S. Senate, said the ratification process should "proceed simultaneously". Obama said he was convinced there would be bipartisan Senate support.

Medvedev reaffirmed Moscow's warning that it could withdraw from the treaty if U.S. missile defence plans undermined the basis for strategic arms control.

Obama's new nuclear strategy document broke with former President George W. Bush's threat of nuclear retaliation in the event of a biological or chemical attack.

The assurance applies only to countries in compliance with the nuclear Non-Proliferation Treaty, so Iran and North Korea would not receive that commitment.

Washington and Moscow have plenty of differences on issues ranging from Iran to missile defence, but the two leaders strove to highlight common ground, including on economic cooperation. However, they did not mention Russia's stalled bid to join the World Trade Organisation.

Obama has put a priority on trying to "reset" relations with Moscow that hit a post-Cold War low during Russia's 2008 war with Georgia, and the treaty could help that.

The successor to the 1991 Strategic Arms Reduction Treaty would limit operationally deployed nuclear warheads to 1,550, down nearly two-thirds from START I.

Later on Thursday, Obama will dine with 11 heads of state from central and eastern Europe. Czech diplomats said the meeting was designed to reassure former Soviet bloc countries that resetting relations with Russia would not diminish U.S. interests in the region.


Agencies