WASHINGTON - The Bush administration has abandoned research into a nuclear "bunker-buster" warhead, deciding instead to pursue a similar device using conventional weaponry, a key Republican senator said Tuesday

Sen. Pete Domenici R-N.M., said funding for the nuclear bunker-buster as part of the Energy Department's fiscal 2006 budget has been dropped at the department's request.

The nuclear bunker-buster had been the focus of intense debate in Congress, with opponents arguing that its development as a tactical nuclear weapon could add to nuclear proliferation.

An administration official, speaking on condition on anonymity because negotiations on the department's spending bill have not yet been completed, confirmed that a decision had been made to concentrate on a nonnuclear bunker-buster.

Administration officials have contended the country must try to develop a nuclear warhead that could destroy deeply buried targets including bunkers tunneled into solid rock. Potential adversaries increasingly are building hardened retreats deep beneath the earth, immune to conventional weapons, the officials said.

But Congress has been cool to the idea of a new nuclear warhead. The House blocked funding for the program, even though the Energy Department had requested only $4 million, scaling back earlier requests.

The Senate approved the $4 million, but a final decision was up to lawmakers working out a compromise between the House and Senate on the department's budget.

Domenici, chairman of the subcommittee that oversees DOE's budget, said the conferees had agreed to drop funding for the program at the request of the department's National Nuclear Security Administration, the agency that oversees nuclear weapons programs.

"The focus will now be with the Defense Department and its research into earth penetrating technology using conventional weaponry," Domenici said in a statement. The NNSA "indicated that this research should evolve around more conventional weapons rather than tactical nuclear devices," the senator said.

"This is a true victory for a more rational nuclear policy," said Stephen Young, a senior analyst for the Union of Concerned Scientists, a nuclear nonproliferation advocacy group. "The proposed weapon, more than 70 times the size of the bomb dropped on Hiroshima, would have caused unparalleled collateral damage."

Last April, a National Academy of Science panel concluded that an earth-penetrating nuclear device would likely cause the same casualties as a surface burst if the weapons are of the same size. Such a bomb could cause from several thousand to 1 million casualties, depending on its yield and location, according to the report requested by Congress.

At a congressional hearing earlier this year, NNSA chief Linton Brooks acknowledged there is no way to avoid significant fallout of radioactive debris from use of a bunker-buster warhead.

He said the administration never intended to suggest "that it was possible to have a bomb that penetrated far enough to trap all fallout. I don't believe the laws of physics will ever let that be true."

Sen. Dianne Feinstein D-Calif., one of Congress' most vocal opponents of the bunker-buster, has said the nuclear bunker-buster "sends the wrong signals to the rest of the world by reopening the nuclear door and beginning the testing and development of a new generation of nuclear weapons."