The US and Russia have finalised a nuclear arms treaty that limits the number of atomic warheads the two former Cold War enemies are allowed to possess.
The New START treaty went into effect when US secretary of state Hillary Clinton exchanged the ratification papers with Russian foreign minister Sergey Lavrov on the sidelines of a security conference in Munich.
The treaty limits each side to 1,550 strategic warheads, down from 2,200. The pact also re-establishes a monitoring system that ended in December 2009 with the expiration of an earlier arms deal.
The treaty was approved by the US Senate in December after US president Barack Obama pressed strongly for its passage. Russia ratified the deal last month.