FACTBOX: U.S.-Russia nuclear arms pacts

(Reuters) - Russia and United States admitted defeat in a bid to agree a new nuclear arms control pact before the START-1 treaty expires on Friday, but pledged to keep working for a new deal to reduce their nuclear arsenals.

Following are key facts on recent arms control treaties:

COLD WAR TREATY:

-- The Strategic Arms Reduction Treaty (START-1), signed in July 1991 by U.S. President George Bush senior and Soviet President Mikhail Gorbachev, led to the largest bilateral nuclear weapons cuts 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-1 was expiring at midnight on Friday, December 4, but will now be extended indefinitely pending agreement on a successor.

START-1 REDUCTIONS:

-- START-1 stipulates that neither side can deploy more than 6,000 nuclear warheads or 1,600 strategic delivery vehicles, which include 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-1 was complicated by the break-up of the Soviet Union. Ukraine, Belarus and Kazakhstan all agreed to transfer their nuclear missiles to Russia.

-- START-1 was the follow-up to the Strategic Arms Limitation Talks (SALT) that dated from 1969-79.

START-2 and SORT:

-- A START-2 treaty, which 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 already be cut much further by 2012 under the still active 2002 Strategic Offensive Reductions Treaty (SORT), also known as the Moscow Treaty.

-- The SORT treaty is weaker than START-1 since it does not offer prescriptive controls for inspection or verification.

NEW TREATY

-- Diplomats are now talking about reaching a new START deal by the year-end.

-- The talks began in April 2009, and received a boost when U.S. President Barack Obama and Russian President Dmitry Medvedev endorsed a swift agreement in July.

-- Obama and Medvedev said the treaty should restrict deployed strategic warheads to between 1,500 and 1,675 while limiting the number of delivery platforms to between 500 and 1,100. But precise details have been kept secret.

-- On November 12, a Russian general warned of differences on how inspections would operate under the new treaty.

(Reporting by Conor Sweeney; Additional writing and editing by Kevin Liffey and David Cutler, London Editorial Reference Unit)