The Pentagon is about to take the first public step toward obtaining a controversial, high-yield, earth-penetrating nuclear weapon that could be aimed at North Korea's underground nuclear and missile production facilities, according to senior Bush administration officials.

Within a week, an Air Force report is to be delivered to the House and Senate Armed Services committees stating the military requirements for the "robust nuclear earth penetrator," a device designed to dig into the ground before it explodes and crushes any facility buried beneath it. Already five times more powerful than the device detonated at Hiroshima, the bomb would have an even greater impact because a nuclear weapon's force is multiplied when its shock wave penetrates the rocky crust of the earth.

Meanwhile, the Pentagon this week sent to Capitol Hill language that would, if approved, lift an eight-year-old congressional restriction on development of a so-called low-yield warhead, one below five kilotons. Such a device would be used to attack facilities holding chemical or biological weapons. In principle, the heat or radiation of the low-yield weapon would destroy the toxicity of the agents before they were spread by the force of the blast.

These moves drew criticism yesterday after Energy Department officials were questioned at a House Armed Services Committee hearing.

Noting the Bush administration's standoff with North Korea over that country's plans to build nuclear weapons, Rep. Ellen Tauscher (D-Calif.), a senior member of the Armed Services panel, said, "I don't see how we look at all the nuclear wannabes in the face when we have announced a half-hearted attempt to take down half our own big nuclear weapons and we are going to now launch ourselves into a whole series of new weapons."

David Albright, a physicist who is president of the Institute for Science and International Security and an expert on North Korea, said, "It is a bad idea to develop these things, which probably would never be used, and do so openly. It develops a lot of paranoia among proliferating states who believe the U.S. is planning to attack them."

When the "earth penetrator" was first discussed in the 1990s, it was conceived as having a low yield -- a relatively small output of radiation, heat and explosive force -- so that if it exploded in the basement of a palace in the outskirts of Baghdad, it would not create much fallout.

Today, however, the goals are different. Potential enemies are hiding their war-making facilities underground, said Everet H. Beckner, deputy administrator for defense programs at the National Nuclear Security Administration, and there is a need for developing a weapon whose nose cone could penetrate frozen soil or rocks.

One of the suspected sites for North Korea's covert uranium enrichment plant is a uranium milling facility built underneath a mountain. Three other suspected nuclear production sites are also hidden near or in large areas carved out of mountains.