FACTBOX: Key facts about U.S.-Russia START I arms deal

(Reuters) - The United States and Russia say finding agreement on a treaty to reduce the number of nuclear weapons will be the main theme of a meeting between the countries' two leaders next week.

Russian media reported on Saturday that Russian President Dmitry Medvedev and President Barack Obama will sign a declaration on a new treaty on April 1.

The nuclear arsenals, which date from the Cold War, are principally covered by the Strategic Arms Reduction Treaty, better known as START I.

The following are key facts on the existing arms control treaties that U.S. President Barack Obama and Russia's Dmitry Medvedev hope will be updated in a new agreement.

COLD WAR WEAPONS TREATY

The START Treaty, signed in July 1991 by U.S. President George Bush and Soviet President Mikhail Gorbachev, led to the largest bilateral reductions of nuclear weapons in history.

It was the result of nearly a decade of sporadic talks between the United States and the Soviet Union in the final years of the Cold War.

START REDUCTIONS

START stipulates that neither side can deploy more than 6,000 nuclear warheads and no more than 1,600 strategic delivery vehicles, which includes intercontinental ballistic missiles, submarines and bomber aircraft.

At the time of agreement, the United States had developed more sophisticated ways to deliver warheads, but the Soviet Union had a larger arsenal of weapons.

Implementation of START was complicated by the break-up of the Soviet Union, although Ukraine, Belarus and Kazakhstan all agreed to transfer their nuclear missiles to Russia.

START was the follow-up to the earlier Strategic Arms Limitation Talks (SALT) that dated from 1969-79.

By 2001, the tightly regulated inspection regime and the targets that START imposed on both the United States and Russia had been met.

AFTER START I

START is due to expire in December 2009, unless both parties agree to either extend it for another five years or can agree on its replacement.

A subsequent START II Treaty that would have restricted the number of independent warheads that can be fixed atop ballistic missiles never entered force.

Active warheads in both arsenals must be cut much further under the still active 2002 Strategic Offensive Reductions Treaty (SORT), also known as the Moscow Treaty. Unlike START, the SORT Treaty does not offer prescriptive controls for either inspection or verification systems.

(Reporting by Conor Sweeney; Editing by Angus MacSwan)