President Barack Obama on Tuesday unveiled a more cautious policy for the US nuclear arsenal, declaring the United States would resort to atomic weapons only 'in extreme circumstances.' Under the new guidelines, the United States for the first time states it would not launch a nuclear attack on a state without atomic weapons that complied with non-proliferation treaty obligations, though Obama said exceptions could be made for 'outliers' Iran and North Korea. '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 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 'modernize' 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 organizations. 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. 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 defense. A senior administration official told selfnews that the upcoming signing of the new START treaty with Russia, the summit, and the Nuclear Posture Review (NPR) will see the administration 'embracing a 21st century approach to nuclear weapons.' 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 doesnt 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 Im going to preserve all the tools that are necessary in order to make sure that the American people are safe and secure.' In April 2009, Obama promised in a landmark speech in Prague to work towards a world without nuclear weapons. But he has admitted he does not expect to see that goal reached during his lifetime. The secretive review of the country's nuclear strategy, which has not been carried out since 2002, has been delayed, bogged down by apparent divisions within the administration. It was initially due to be released in December. The report, which runs about 80 pages, was organized around five key objectives, a senior US defense official said. These include US 'declaratory policy,' meaning the conditions under which the United States might use nuclear weapons. The report also will stress 'our sense of urgency about proliferation and terrorism,' address the nuclear balance with Russia and China, offer assurances to US allies 'in a changing security environment,' and ensure that the US arsenal is safe, secure and effective, the defense official said. 'All countries, whether theyre nuclear weapons states, non-nuclear weapons states or aspiring nuclear weapons states, I think should be very clear about what our approach and our strategy is,' Obama told the Times. 'And I do think that when youre looking at outliers like Iran or North Korea, they should see that over the course of the last year and a half we have been executing a policy that will increasingly isolate them so long as they are operating outside of accepted international norms.' The United States has said it currently has some 2,200 nuclear warheads, while Russia is believed to have about 3,000. 