Backgrounder: U.S.-Russia treaties on strategic arms reductions

BEIJING, April 7 (Xinhua) -- Russia and the United States will sign a new arms reduction treaty on Thursday in Prague, the successor to the expired 1991 Strategic Arms Reduction Treaty (START I).

Since the 1980s, the United States and Russia (and its predecessor the former Soviet Union) have held rounds of talks on strategic disarmament and have signed several treaties.

In July 1991, the United States and the former Soviet Union signed the START I, which barred its signatories from deploying more than 6,000 nuclear warheads on a total of 1,600 intercontinental ballistic missiles, submarine-launched ballistic missiles, and bombers.

The treaty took effect in December 1994 and expired on Dec.5 last year.

In January 1993, the United States and Russia signed START II, which would stipulate that the U.S. should reduce its total deployed strategic nuclear warheads to 3,500 while Russia cut its arsenal to 3,000. The treaty would also have banned the use of multiple-warheads on ICBMs.

However, the treaty, although ratified, has never entered into force.

In May 2005, then U.S. President George W. Bush and his Russian counterpart Vladimir Putin signed the Strategic Offensive Reductions Treaty (SORT), or the Moscow Treaty, under which the two countries will slash their arsenals to 1,700 to 2,200 deployed strategic nuclear warheads each. The treaty took effect in June 2003, with the expiration date set for Dec. 31, 2012.

In April 2009, U.S. President Barack Obama met his Russian counterpart Dmitry Medvedev in London. Both agreed to immediately begin bilateral intergovernmental negotiations to work out a "new, comprehensive, legally binding agreement" to replace START I.

The two sides had sought to finish negotiations before START I expired on Dec. 5 2009. However, despite rounds of negotiations, they failed to reach a new pact. U.S. missile defense plans in east Europe were a major sticking point in the nuclear disarmament negotiations, apart from counting methods and verification procedures.

In 2010, negotiators from the two countries were engaged in tough talks in Switzerland to hammer out a new pact. The outline of the new treaty, agreed upon by both presidents, includes slashing nuclear warheads to 1,500-1,675 and delivery vehicles to 500-1,000.

On March 26, Obama and Medvedev held a telephone conversation and both agreed to sign the new treaty on April 8 in Prague.