The US and Russian leaders signed the New Start treaty at a ceremony attended by hundreds of officials in the lavishly decorated Spanish Hall of Prague Castle, the Czech president's residence.

Under the treaty, the two powers are committed to reducing the number of deployed strategic warheads to 1,550 each or 30% lower than the 2002 Moscow Treaty.

The pact also limits the number of deployed delivery vehicles - ballistic missiles and heavy bombers - to no more than 700. However, each bomber counts as one warhead irrespective of the fact that it might carry multiple bombs or missiles.

Mr Obama said it is an important milestone, but "just one step on a longer journey" of nuclear disarmament.

Mr Medvedev said the deal will create safer conditions throughout the world.

If ratified by lawmakers in both countries, the treaty will replace the Strategic Arms Reduction Treaty (Start) of 1991, which has expired.