"For generations, the United States' nuclear deterrent has helped prevent proliferation by providing our non-nuclear allies in Nato, the Pacific, and elsewhere with reassurance and security," Clinton said.

"The policies outlined in this review will allow us to continue that stabilising role."

The revised Nuclear Posture Review - a document required by congress of every American president - stipulates that nuclear weapons be "fundamentally" used to deter or respond to a nuclear attack on US soil.

'Play by the rules'

Gates said that the new policy sends a strong message to Iran and North Korea to "play by the rules" and make clear that "all options are on the table" for those not following the NPT.

in depth

  Factfile: The world's nuclear stockpile
  Video: Obama's nuclear doctrine
  Obama on US-Russia treaty
  Inside Story: A world without atomic weapons
  Riz Khan: Global nuclear disarmament

In an interview with the New York Times on Monday, Barack Obama, the US president, said non-nuclear threats could be deterred with "a series of graded options", a combination of old and newly designed conventional weapons.

The White House said substantial new US investments in the weapons laboratories and other technological undergirdings of the nuclear arsenal would "facilitate further nuclear reductions", and extend the life of warheads currently in the nuclear force.

"This is an alternative to developing new nuclear weapons, which we reject," the White House statement said.

The much-anticipated announcement on the size and role of the US nuclear weapons stockpile is aimed at urging Russia to return to the bargaining table following senate ratification of the new Start arms reduction treaty.

The deal is to be signed by Obama and Dmitry Medvedev, the Russian president, in Prague in the Czech Republic on Thursday, before a nuclear security summit in Washington next week.

The US says investments in nuclear labs will 'facilitate further nuclear reductions' [EPA]

The White House hopes to overcome Russia's expressed reluctance to move beyond Start, especially if it means cutting Moscow's arsenal of tactical, or short-range nuclear arms.

Reducing the short-range bombs and stored warheads would involve more intrusive inspections than agreed to in the treaty Obama and Medvedev would sign this week.

Another potential obstacle to expanding the next set of nuclear arms talks is Russia's strong resistance to US missile defence in Europe.

Moscow sought to include constraints on missile defence in the new Start, but US officials say the agreement contains no such limits.

Russia gave warning on Tuesday that it may quit the pact if US missile defence plans went too far.

Lavrov's threat

Sergei Lavrov, Russia's foreign minister, acknowledged that the new Start "reflects a new level of trust between Moscow and Washington".

However, as the pact placed no restrictions on either side developing and deploying new missile defences, he cautioned that US moves to do so could provide grounds for Russia to quit the treaty unilaterally.

Lavrov said Russia could quit the new Start if US missile defence plans went too far [AFP]

"Russia will have the right to abandon the Start treaty if a quantitative and qualitative buildup of the US strategic anti-missile potential begins to significantly affect the efficiency of Russia's strategic nuclear forces," Lavrov said.

The US has an estimated 200 short-range nuclear weapons in Europe under a Nato agreement, whereas the Russians are believed to have 10 times that many deployed in European Russia.

Russia, on the other hand, sees tactical nuclear warheads as a counterweight to the military superiority of Nato.

These weapons are a legacy of the Cold War standoff in divided Europe, and there is now a growing push by Europeans to negotiate away these weapons.

The review is a test of Obama's effort to make controlling nuclear arms worldwide one of his signature foreign policy initiatives. It is also important because it will affect defence budgets and weapons deployment for years to come.

The strategy was developed after a lengthy debate among Obama's aides and military officials over whether to declare that the US would never be the first to use nuclear weapons in a crisis but would act only in response to attack.