FACTBOX-Who are the world's nuclear powers?

(Reuters) - The presidents of Russia and the United States, the world's two biggest nuclear powers, will hold their first summit in Moscow next week.

U.S. President Barack Obama and Russian President Dmitry Medvedev are trying to forge a more pragmatic relationship which could eventually lead to deep cuts in the number of atomic weapons deployed in both their arsenals.

Here are some details of official and unofficial nuclear powers and their weapon stocks:

OFFICIAL NUCLEAR POWERS:

* UNITED STATES: According to the counting rules in the Strategic Arms Reduction Treaty (START), as of January 2009 the United States had an estimated 5,200 nuclear warheads and 2,700 operationally deployed warheads (2,200 strategic and 500 nonstrategic warheads). The distinction between strategic and nonstrategic (also known as tactical) nuclear weapons reflects the military definitions of, on the one hand, a longer-term strategic mission and, on the other hand, the more immediate tactical use of nuclear weapons.

-- According to the May 2002 Treaty of Moscow (the Strategic Offensive Reductions Treaty, or SORT) between the United States and the Russian Federation, both are required to reduce their deployed strategic nuclear forces to 1,700-2,200 warheads by 2012.

* RUSSIA: Russia is estimated to have around 14,000 nuclear weapons, although the total stockpile size is uncertain because there is no accurate count of tactical nuclear weapons. Under the provisions of START I, the Russian nuclear arsenal has been reduced to approximately 4,138 strategic nuclear warheads, as of July 2008.

* FRANCE: France has been a party to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) since 1992. In 2008, President Nicolas Sarkozy announced the country would leave its submarine missile arsenal in place while cutting its stock of air-launched weapons by a third to around 290 warheads.

-- As of September 2008, France had already pared down its arsenal to approximately 300 nuclear warheads.