President Barack Obama plans "dramatic reductions" in the country's nuclear arsenal, a senior US official said Monday, but it remains unclear if he will opt for a radical break from past policy.

A review of nuclear strategy underway "will point to dramatic reductions in the stockpile, while maintaining a strong and reliable deterrent through the investments that have been made in the budget," a senior administration official told AFP.

The review, due to be completed later this month, will also "point to a greater role for conventional weapons in deterrence" and rule out the need to develop low-yield "bunker-buster" nuclear weapons for penetrating underground targets, said the official, speaking on condition of anonymity.

Defense Secretary Robert Gates 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 former CIA director, has been portrayed by arms control advocates as reluctant to back major changes in nuclear arms policy.

It remains unclear how Obama will decide the crucial question of whether the United States should openly declare the conditions for the possible use of nuclear weapons, or retain ambiguous language.

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

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

Amid an intense debate among Obama's advisers, arms control experts and media reports say such a shift appears unlikely and the Obama may back only modest policy changes.

Accounts of the long-delayed policy review suggest "a very conventional document that will fall far short of the president's rhetoric," Jeffrey Lewis, recently wrote on ArmsControlWonk.com.

The effort likely will produce "a very status-quo document," said Lewis, a leading expert on proliferation.

In April, Obama promised in a speech in Prague to work towards a world without nuclear weapons and to put an end to "Cold War thinking" in US strategy.

He has called for nuclear powers to make major cuts in stockpiles in return for stepped up global efforts to counter the spread of atomic weapons.

During the Cold War, US nuclear strategy focused on two potential enemies, the Soviet Union and China.

Current strategy assumes six possible adversaries -- China, Iran, North Korea, Russia, Syria and a terrorist organization with weapons of mass destruction, according an analysis of strategy documents by the Federation of American Scientists (FAS), a Washington-based non-partisan group.

Three of those possible "adversaries" do not have nuclear arms and two have signed on to the nuclear non-proliferation treaty.

A more restrictive policy on the use of nuclear weapons would mean rewriting the current US nuclear strategy, the FAS said.

Obama also must weigh whether to withdraw remaining shorter-range, "tactical" nuclear weapons from several NATO bases in Europe, an issue his deputies have reportedly raised with allies.

Amid concern over Iranian and North Korean nuclear programs, the Obama administration is pushing to bolster the nuclear non-proliferation treaty, which comes up for review this year.

Washington's policy review comes as the United States and Russia appear close to a new deal to slash their nuclear arsenals, despite Moscow's concerns about Washington's latest missile defense plans.

The broad outlines of a new treaty on nuclear weapons have been clear since a summit in July, when Obama and his Russian counterpart Dmitry Medvedev agreed to slash 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.

The administration is also pushing the Senate to ratify the Comprehensive Test Ban Treaty, which would ban all nuclear tests, whether military or civilian.