It will also rule out the need to develop low-yield "bunker-buster" nuclear
weapons for penetrating underground targets, the official added.

Robert Gates, the US defence secretary, was expected on Monday to present final options to Obama on the long-delayed Nuclear Posture Review, which was initially supposed to be released in December.

Gates, an influential figure in Obama's cabinet and a former CIA director, has been portrayed by arms control advocates as reluctant to back major changes in nuclear arms policy.

Cold War thinking

The planned scaling down of the US nuclear arsenal comes almost a year after Obama promised in a speech in the Czech capital, Prague, to work towards a world without nuclear weapons.

Obama, who has called for nuclear powers to make major cuts in stockpiles as part of global efforts to counter the spread of atomic weapons, also promised to put an end to "Cold War thinking" in US strategy.

The president is also said to be considering weighing the option of whether to withdraw remaining shorter-range, "tactical" nuclear weapons from several Nato bases in Europe, an issue his deputies have reportedly raised with allies.

Some of Obama's allies in Congress are advocating a change in US policy that permits use of nuclear weapons in response to a biological or chemical attack, even against a country without an atomic bomb.

Modest changes

The legislators want Obama to declare that the exclusive purpose of the arsenal is to deter nuclear attack, a move that would allow for more drastic cuts in the arsenal.

But arms control experts and media reports suggest such a shift appears unlikely and that Obama may back only modest policy changes.

The Obama administration, still grappling with Iranian and North Korean nuclear programmes, is pushing to bolster the nuclear Non-Proliferation Treaty, which comes up for review this year.

The broad outlines of a new treaty on nuclear weapons have been clear since a summit in July, when Obama and Dmitry Medvedev, his Russian counterpart, agreed to reduce the number of warheads on either side to between 1,500 and 1,675.

The presidents also agreed that the number of carriers capable of delivering the warheads should be limited to between 500 and 1,100.