By Jonathan Marcus BBC defence correspondent

The United States formally withdrew from the Anti-Ballistic Missile (ABM) treaty on Thursday.

Mr Putin called the US decision a "mistake"

The ABM agreement was signed in 1972 by the US and the then Soviet Union.

Its intention was to maintain the effectiveness of nuclear deterrence by strictly limiting the deployment of anti-missile systems.

But the Bush administration came into office determined to abandon the treaty which it saw as constraining its ability to test new anti-missile defences.

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But even without the ABM treaty an American missile defence system is still a long way away.

Rogue states argument

For President George W Bush the ABM treaty is simply an outdated relic of the Cold War.

Experts say US is "decades away" from the proposed systems

The US view is that the main missile threat comes not from an exchange between major nuclear-armed powers but from the much smaller arsenals of so-called rogue states like Iraq or North Korea.

The Bush team believes that effective defences can be built to intercept such limited attacks.

But the ABM treaty, in their view, limited the sort of testing that could be done and hence it had to go.

Arms control experts warned that abandoning such a treaty would set a dangerous precedent and that it could seriously undermine wider arms control efforts.

'Article of faith'

In the event both Russia and China did not like America's decision when it was first announced last December.

But they probably realised that missile defence has become an article of faith in the Pentagon and that the Americans were determined to push ahead with their anti-missile research.

Support in the administration for missile defence has grown even stronger in the wake of the 11 September attacks.

This is despite the fact that many experts believe that the real nuclear or biological threat may come not from a missile but from a weapon smuggled into the US or even one that was actually put together there.

'Decades away'

With the ABM treaty out of the way the US research effort will no doubt be expanded.

But progress so far has been very slow. Existing tests have had only mixed results.

The Pentagon is planning to build an experimental interceptor site in Alaska by 2004.

But leading scientists believe that the US is decades away from the sort of multi-layered defensive system proposed by President Bush.

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