Instead of making such a warning from time to time as threats arise, the Bush administration "has set it out as a general principle, and backed it up by explaining what has changed in the world," Smith said.

In a policy statement issued only last month, the White House said the United States "will continue to make clear that it reserves the right to respond with overwhelming force -- including through resort to all of our options -- to the use of weapons of mass destruction against the United States."

One year ago, the administration completed a classified Nuclear Posture Review that said nuclear weapons should be considered against targets able to withstand conventional attack; in retaliation for an attack with nuclear, chemical or biological weapons; or "in the event of surprising military developments." And it identified seven countries -- China, Russia, Iraq, North Korea, Iran, Libya and Syria -- as possible targets.

The same report called on the government to develop smaller nuclear weapons for possible use in some battlefield situations. The United States and Russia have stockpiles of such tactical weapons, which are often small enough to be carried by one or two people yet can exceed the power of the bomb that destroyed Hiroshima, Japan.

The administration has since been pushing Congress to pay for a study of how to build a smaller, more effective version of a 6-year-old nuclear bunker-buster bomb, the B-61 Mod 11. Critics maintain the administration's eagerness for this study shows officials' desire to move toward building new weapons and to end the decade-old voluntary freeze on nuclear testing.

The B-61 is considered ineffective because it can burrow only 20 feet before detonating. The increasingly sophisticated underground command posts and weapon storage facilities being built by some countries are far deeper than that. And the closer to the surface a nuclear device explodes, the greater the risk of the spread of radiation.

The reported yield of B-61 devices in U.S. inventory varies from less than 1 kiloton of TNT to more than 350. The Hiroshima bomb was between 10 and 15.

Discussion of new weapons has set off a heated argument among experts on the value and effects of smaller-yield nuclear weapons.

Some Pentagon officials contend that the nation could develop nuclear weapons that could burrow deep enough to destroy hardened targets. But some independent physicists have argued that such a device would barely penetrate the surface while blowing out huge amounts of radioactive dirt that would pollute the region around it with a deadly fallout.

Wade Boese of the Arms Control Assn. in Washington said there is no evidence that conventional arms wouldn't be just as effective in reaching deeply buried targets.