US President Barack Obama and the Russian leader, Dmitry Medvedev, have signed a landmark nuclear arms treaty in the Czech capital, Prague.

The treaty commits the former Cold War enemies to each reduce the number of deployed strategic warheads to 1 550 - 30% lower than the previous ceiling, the BBC reported.

It also limits the number of deployed "launchers" - ballistic missiles and heavy bombers - to no more than 700.

The deal replaces the expired Strategic Arms Reduction Treaty (Start) of 1991.

After it was agreed, Obama hailed the deal as the most comprehensive weapons control agreement in nearly two decades.

"With this agreement, the United States and Russia - the two largest nuclear powers in the world - also send a clear signal that we intend to lead," he said.

The treaty must be ratified by the US Senate and the Russian Duma.

The US and Russian leaders signed the New Start treaty at 13:00 Bulgarian time in Prague Castle, the Czech president's residence.

On Tuesday, Russian Foreign Minister Sergei Lavrov said the New Start treaty marked a "new level of trust" between the countries. He said the original Start treaty, which expired in December, was "born from the Cold War" and contained much that was "discriminatory" towards Russia.

Lavrov noted that the new pact explicitly acknowledged a direct link between offensive nuclear weapons and missile defense systems, and warned that his country could opt out if it felt threatened by US plans.

"Russia will have the right to abandon the Start treaty if a quantitative and qualitative build-up of the US strategic anti-missile potential begins to significantly affect the efficiency of Russia's strategic forces," he added.

It was Moscow's concerns over Washington's plans to base interceptor missiles in Poland and a radar station in the Czech Republic that helped delay the new treaty. President Obama shelved the idea in September.

Lavrov said Washington's current plans - which include ground-based interceptor missiles in Romania and the possibility of others in Bulgaria - seemed acceptable.