Washington ? As the Pentagon continues a highly visible buildup of troops and weapons in the Persian Gulf, it is also quietly preparing for the possible use of nuclear weapons in the potential war against Iraq, according to a report by a defense analyst.

Although they consider such a strike unlikely, military planners have been actively studying lists of potential targets and considering options, including the possible use of so-called “bunker buster” nuclear weapons against deeply buried military targets, says analyst William M. Arkin, who writes a regular column on defense matters for the Los Angeles Times.

Military officials have been focusing their planning on the use of nuclear arms in retaliation for a strike by the Iraqis with chemical or biological weapons, or to pre-empt one, Arkin says.

Administration officials believe that in some circumstances, nuclear arms may offer the only way to destroy deeply buried targets that may contain unconventional weapons that could kill thousands.

Some officials have argued that the blast and radiation effects of such strikes would be limited.

But that is in dispute. Critics contend that a bunker-buster strike could involve a huge radiation release and dangerous blast damage.

They also say that use of a nuclear weapon in such circumstances would encourage other nuclear-armed countries to consider using those weapons in more kinds of situations, and would badly undermine the half-century effort to contain the spread of nuclear weapons.

Although it may be highly unlikely that the Bush administration would authorize the use of such weapons in Iraq — Arkin describes it as a worst-case scenario — the mere disclosure of its planning contingencies could stiffen the opposition of France, Germany and Middle East nations to any invasion of Iraq.

“If the United States dropped a bomb on an Arab country, it might be a military success, but it would be a diplomatic, political and strategic disaster,” said Joseph Cirincione, director of nonproliferation studies at the Carnegie Endowment for International Peace in Washington.

He said there is a danger of the misuse of a nuclear weapon in Iraq because of the chance that “somebody could be seduced into the mistaken idea that you could use a nuclear weapon with minimal collateral damage and political damage.”

Iraq's neighboring nations fear the possible effects of a U.S. war to oust Saddam Hussein. Protesters gathered Jan. 19 in Ankara, Turkey, to show their opposition to a U.S. strike. One military expert says the United States is preparing plans for the possible use of nuclear weapons in such an attack.

In the past year, Bush administration officials have repeatedly made clear that they want to be better prepared to consider the nuclear option against the threat of “weapons of mass destruction” in the hands of terrorists and rogue nations. The current planning activities, as reported by Arkin, offer a concrete example of their determination to follow through on this pledge.

Arkin also says the Pentagon has changed the bureaucratic oversight of nuclear weapons so that they are no longer treated as a special category of arms, but grouped with conventional military options.

A White House spokesman declined comment Friday on Arkin’s report, except to say that “the United States reserves the right to defend itself and its allies by whatever means necessary.”

Consideration of the nuclear option has defenders.

David J. Smith, an arms-control negotiator in the first Bush administration, said presidents would only consider using such a weapon “in terribly ugly situations where there are no easy ways out. If there’s a threat that could involve huge numbers of American lives, I as a citizen would want the president to consider that option.”