Washington ? As the Pentagon looks for ways to neutralize chemical or biological weapons underground, it is considering development of a warhead that would surround them with a hard or sticky foam rather than blow them up.

Another possibility is a nonexploding warhead that spreads flammable materials to incinerate biological agents.

Both approaches are still on the drawing board. They would be alternatives to conventional high-explosive warheads, which might allow contaminants to escape, threatening civilians or U.S. troops.

“It’s not as simple as blowing it up,” said Stephen Younger, director of the Defense Threat Reduction Agency, a Pentagon office charged with assessing and countering weapons of mass destruction.

David Wright, a weapons expert at the Union of Concerned Scientists, said he had not heard of the concept of attacking chemical or biological agents with foams or flammable materials. He said, however, that it seemed questionable whether such warheads could penetrate far enough into buried facilities.

In an interview Wednesday with a group of reporters, Younger said that if a facility containing the chemical or biological agents was large enough that U.S. commanders decided it had to be destroyed rather than temporarily neutralized, then a deep-penetrating nuclear weapon might be used.

The Pentagon has asked Congress for $15 million in its 2003 budget to study such a weapon.

The question of how to neutralize or destroy bunkers containing chemical, biological or nuclear materials has gained urgency as the Bush administration contemplates the possibility of using military force to topple Iraqi President Saddam Hussein.

Prior to the 1991 Gulf War, Iraq was actively developing nuclear weapons and possessed chemical and biological agents that could be used as weapons.