If either state were to neutralise the threat from the other - by building a national ABM system - it would no longer have to worry about retaliation, and would be more likely to launch a first strike.

Mr Putin has warned in the past that if the US violates the ABM treaty unilaterally, Moscow will consider all other arms control agreements null and void.

But the first country to be affected by US plans would be China, whose arsenal of ballistic missiles is so small that it would be powerless against the limited NMD currently under consideration in Washington.

China's response

Russian disarmament experts say that China would inevitably respond by building more missiles, in order to improve the chance of penetrating US defences.

It has also been suggested that other countries, such as India and Pakistan, could follow China's lead.

But Russia also fears that a limited NMD system could serve as the basis for a larger one - already actively promoted by Republicans in the US - which would undermine the Russian nuclear deterrent too.

"What is actually meant here is a system with controls and targeting abilities, some of them space-based, of a kind that can easily be expanded to national dimensions," said the hawkish General Leonid Ivashov, head of the Russian Defence Ministry's department for international co-operation.

Russia's arsenal of missiles and warheads is already being cut back, in line with existing disarmament treaties, but for purely financial reasons it is expected to fall below the limit of 2,000 to 2,500 warheads currently envisaged in a future strategic disarmament treaty known as Start 3.

Budgets

The cost of maintaining thousands of nuclear warheads places an enormous strain on Russia's limited military budget.

The Russian military is deeply sceptical that the US could design a watertight missile defence system, but it would be forced to assume otherwise unless or until it was put to the test during an actual missile attack.

Russian officials have said that one of their first responses to any US abrogation of the ABM treaty would be to put multiple warheads and decoys on ballistic missiles.

Some have also argued that the US is overstating the threat from what it calls rogue states, such as North Korea.

Foreign Minister Igor Ivanov said in a recent interview: "I do not think that the desire to amend [the ABM treaty] is prompted by any real threats to America from anyone. It is connected with the interests of the US military-industrial complex."

The nature of the threat is one more issue that Russia wants to discuss and analyse jointly with the US and Europe.

Meanwhile, Russia's military-industrial complex is also hungry for money.

One Russian writer on defence issues, Pavel Felgenhauer, has said that some generals are actually hoping that the US will go ahead with NMD, in order to ensure a big new wave of spending on the country's defence.