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

(Reuters) - Russia and the United States hold talks on Tuesday aimed at finding a replacement for a Cold War arms reduction treaty that curbs nuclear arsenals.

Russia and the United States said last month they would start talks on a new deal to cut nuclear warheads.

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

The following are key facts on arms control treaties:.

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

-- The implementation of START was complicated by the break-up of the Soviet Union although Ukraine, Belarus and Kazakhstan 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 targets that START imposed on 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 a successor.

-- A START II Treaty that would have restricted the number of independent warheads that can be fixed to ballistic missiles never entered into 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; Additional writing and editing by David Cutler, London Editorial Reference Unit)