The head of the U.S. Strategic Command, which controls the nation's nuclear weapons delivery systems, told Congress yesterday that future global strike plans would increasingly involve advanced conventional weapons.

"This innovative approach will enable the command to deliberately and adaptively plan and rapidly deliver limited-duration, nonnuclear combat power anywhere in the world," Adm. James O. Ellis told a Senate Armed Services subcommittee yesterday.

Conventional and "non-kinetic" weapons, which put out electronic pulses as their kill mechanism, along with special operations capabilities for precision guidance, will be incorporated "into the nation's strategic war plan to further reduce our reliance on nuclear weapons," Ellis said.

He said the need for nuclear weapons will continue, for example, to be able to hit "hard and deeply buried targets" in rock formations "that can't be easily penetrated by conventional weapons."

Although the Strategic Command's new emphasis on conventional weapons was a highlight at yesterday's hearing, the administration's new nuclear strategy, which incorporates preemption as a doctrine, led to questions about plans for what could become nuclear weapons of the future.

Linton F. Brooks, acting head of the National Nuclear Security Administration, which runs the nation's nuclear weapons complex, told senators that a Pentagon study on the potential for a new weapon to go after deeply buried targets, the Robust Nuclear Earth Penetrator, was delivered last month. That triggered release of $15 million to begin feasibility and cost studies later this month.

The studies, to be done at two nuclear weapons laboratories, would look at adapting one of two existing nuclear warheads by putting a harder casing around it and creating a fusing that could survive digging into the ground before detonating. Using existing warheads would eliminate the need for new underground testing, thus maintaining the testing moratorium that went into effect in 1992 under the previous Bush administration.

In addition, Brooks said, $6 million would be spent on studying advanced concepts for future nuclear weapons, "which someday may be needed." He tried to reassure senators that the studies would not automatically lead to production of new weapons without further congressional approval.

But Brooks ran into opposition from Sens. Jack Reed (D-R.I.) and Bill Nelson (D-Fla.) when he pressed the administration's desire to repeal a nine-year legislative ban on research and development that could lead to production of any new low-yield nuclear warhead below 5 kilotons (explosive power equal to 5,000 tons of TNT).

Brooks said the prohibition had "a chilling effect" on advanced nuclear design studies and was "an artificial intellectual restraint." Reed countered that its repeal would hurt efforts at nonproliferation by sending a signal to other countries that the United States, with a full arsenal of nuclear weapons, was looking for new ones.

Among the concepts being studied is a weapon that could be used against stores of chemical or biological weapons. According to Brooks, it could be used "against a particular set of biological agents where a large burst of radiation could be used to kill such bugs."