Q+A: Why are U.S.-Russia nuclear talks important?

(Reuters) - Russian Prime Minister Vladimir Putin said on Tuesday U.S. plans for a missile defense system were the main obstacle to reaching a new deal on reducing Cold War arsenals of nuclear weapons.

The two largest nuclear powers say they are close to agreeing on a successor to the 1991 Strategic Arms Reduction Treaty (START I), although U.S. President Barack Obama and Russian President Dmitry Medvedev have yet to clinch a deal.

The following are key issues about the successor agreement:

* WHAT IS IT?

- Presidents Obama and Medvedev in April said they wanted to agree a new deal by the December 5 expiry date of START I but talks snagged and the two sides agreed to extend the old treaty until a replacement was ready.

The two leaders agreed in July that a new treaty would limit operationally deployed nuclear warheads to 1,500-1,675, cutting by a third from the current levels of 2,200-2,790.

Those limits seem impressive but they do not go far beyond the 2002 Strategic Offensive Reductions Treaty (SORT), also known as the Moscow Treaty, under which each side agreed to cut strategic nuclear warheads to 1,700-2,200 by 2012.

Obama and Medvedev said the provisional limit for delivery systems -- jargon for the bombers, missiles and launch systems that deliver a warhead to a target -- should be within the 500-1,100 range.

* WHY IS A NEW DEAL IMPORTANT?

- Washington and Moscow say finding a replacement for START I would help "reset" relations after a period of tension. Obama has said improving relations with Russia -- a key player in Afghanistan and Iran -- is one of his priorities.

START I played an important role in reducing the superpower brinkmanship of the Cold War and both the United States and Russia -- which hold 95 percent of the world's nuclear arms -- are committed to reducing the number of atomic weapons.

Moscow and Washington realize their nuclear superiority is not threatened by any other power so, officials say, it makes sense to get rid of more weapons.

Russia's vast store of Soviet-era nuclear weapons is one of the factors keeping Moscow at the top table of world politics. After Russia's conventional forces were starved of cash in the chaos that followed the fall of the Soviet Union in 1991, its still mighty nuclear deterrent is the centerpiece of the Kremlin's military doctrine.

If the new treaty imposes deep cuts and strict verification procedures, it would become the most significant arms reduction agreement between Russia and the United States since the collapse of the Soviet Union.

A new deal is needed to impose tough verification procedures which ensured that cuts could be checked. START I had strict inspection clauses but the 2002 Moscow Treaty did not contain them, cited by arms experts as a fundamental flaw.

The verification procedure is important because it allows the former Cold War foes accurately to predict how many weapons each side has and thus reduces the chance of a new arms race.

A replacement for START I is seen as the first step toward much deeper cuts. Both sides hope an agreement on START could lay the ground for more ambitious talks about reducing the silos of thousands of non-deployed nuclear warheads and shorter-range tactical nuclear warheads.

Those talks could also impose must bigger cuts to deployed strategic warheads and delivery systems.

A deep cut by the world's two biggest nuclear powers could help create the momentum for a conference next year on revising the 1970 Non-Proliferation Treaty.

* WHAT ARE THE PROBLEMS?

- The two sides have agreed on a news blackout from negotiations in Geneva, so there have been few details about what has caused the delay in achieving a deal.

Russia says U.S. plans for a missile defense system have complicated talks. Moscow says the initial plans of George W. Bush would have undermined Russian security, although Washington says the shield is a defense against a possible attack by Iran.

Working out how to count nuclear weapons seems simple at first glance, but can be exhaustingly difficult. For example, if a missile carries 10 warheads, should it be one weapon or 10? How does one count a missile that can carries 10 warheads, but has only one warhead currently deployed on it?

Russia wants to see the number of delivery systems to be further limited. However, Moscow has worries about such systems which have had their warheads removed but which could swiftly be deployed in the event of conflict.

Verification procedures under START were extremely strict and intrusive, but both sides have raised concerns about the complexity of the system. Russian Foreign Minister Sergei Lavrov said on Dec 17 he wanted cheaper verification procedures.

Russia is concerned that the United States may simply increase the number of its new conventional weapons, some of which can be almost as destructive as nuclear bombs.

(Reporting by Guy Faulconbridge in Moscow; editing by Andrew Dobbie)