WASHINGTON, USA (AFP) -- The US administration yesterday unveiled new guidelines for the nation's nuclear arsenal, saying it would not attack non-nuclear states with atomic arms which would only be used "in extreme circumstances".

In a shift of policy, the United States said for the first time that states without atomic weapons which complied with non-proliferation treaty obligations need not fear a US nuclear attack.

But President Barack Obama warned exceptions could be made for "outliers" Iran and North Korea, both of which are suspected of trying to acquire a nuclear bomb.

"Indeed, the United States wishes to stress that it would only consider the use of nuclear weapons in extreme circumstances to defend the vital interests of the United States or its allies and partners," the administration said in a policy document.

The Nuclear Posture Review released yesterday also describes "nuclear terrorism" as an immediate and extreme threat, with efforts to prevent the spread of atomic weapons and material a top priority.

The policy rules out building new nuclear weapons, but does call for setting aside billions of dollars to "modernise" existing US weaponry.

Washington has never renounced the "first use" of nuclear weapons, and Obama's policy stops short of calls by arms control activists to explicitly limit their role to deterrence of other nuclear-armed states or terrorist organisations.

In an interview with The New York Times, Obama said his approach seeks to eliminate Cold War ambiguities about when such weapons could be used.

Obama's new take on nuclear policy comes two days before he signs a treaty with Russia to slash stockpiles of long-range nuclear warheads by a third, and less than a week before he hosts world leaders at a key nuclear summit in Washington.

A senior administration official told AFP the upcoming signing of the new START treaty with Russia, the summit, and the Nuclear Posture Review will see the administration "embracing a 21st century approach to nuclear weapons".

In Moscow, Russian Foreign Minister Sergei Lavrov said the new START treaty "reflects a new level of trust between Moscow and Washington", but he warned Russia could withdraw unilaterally if Washington moves to far on missile defence.

In order to pursue a key foreign policy aim of halting nuclear proliferation, Obama has committed the United States -- the only country ever to unleash an wartime atomic bomb -- to a series of nuclear arms cuts.

"Now, the Nuclear Posture Review states very clearly, if you are a non-nuclear weapons state that is compliant with the NPT, you have a negative assurance we will not be using nuclear weapons against you," he told The Times.

"That doesn't mean that you might not engage in some actions that are profoundly detrimental to US national security, which require action on our part," Obama said.

"And I'm going to preserve all the tools that are necessary in order to make sure that the American people are safe and secure."