The United States secretary of state Hillary Clinton has launched a landmark nuclear arms reduction pact with Russia, a showpiece of Washington's “reset” of ties with its former Cold War enemy. The new START nuclear arms reduction treaty officially came into force when Ms Clinton and Russian counterpart Sergei Lavrov exchanged ratification documents at a security conference in the German city of Munich. The pact slashes existing warhead ceilings by 30 per cent over the next 10 years and limits each side to 700 deployed long-range missiles and heavy bombers. The original 1991 pact expired at the end of 2009 amid stark differences over how the two sides planned to proceed. The United States and Russia possess 90 per cent of the world's nuclear weapons.