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

Under its terms, the two countries must reduce the number of their nuclear warheads to 1,550 each - a 30 percent reduction.

The treaty took months of difficult negotiations which were complicated by US plans to build a missile defence shield in Europe. The new nuclear treaty is to replace the 1991 Strategic Arms Reduction Treaty. The new START treaty has yet to be ratified by the US Congress and the Russian parliament.