The Pentagon is looking for a new type of nuclear weapon capable of attacking the Soviet Union's newly deployed mobile intercontinental ballistic missiles, according to previously secret Pentagon testimony to Congress last March.

Mobile ICBM systems, according to testimony by a nuclear scientist who has since left the government, "create the biggest question in strategic forces . . . . The Soviets are reducing the effectiveness of our present strategic weapons and we are looking at a variety of approaches to be able to hold these offensive forces at risk."

Richard L. Wagner Jr., who was then special assistant to Defense Secretary Caspar W. Weinberger for atomic weapons, told a House Appropriations subcommittee last March that "strategic relocatable target studies will attempt to identify effective nuclear weapons concepts to attack mobile targets with our forces."

He said results of these studies will be made part of a broader Air Force survey and that development of any new nuclear weapons concept would depend on its expected effectiveness and cost.

The testimony was released by the subcommittee.

In another hearing earlier this year, Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, told the Senate Foreign Relations Committee that if an arms control agreement led to a sharp reduction in weapons, the United States would have to pursue new nuclear designs to keep the remaining Soviet force at risk.

The search for a new nuclear system for mobile targets marks the beginning of what could be the next round of warhead research. It also guarantees continuation of the debate over whether the United States should negotiate to ban all nuclear weapons tests. A new warhead could not be developed without testing.

The Soviets have deployed 72 new road-mobile SS25 ICBMs and are expected to have a railroad-mobile SS24 operational later this year, according to Pentagon sources.

A former Pentagon officer who remains a key adviser to the Air Force said that studying "relocatable targets has become a hot item," but added that he was mystified by the search for a new nuclear weapon. "The problem is finding them the Soviet missiles ," he said. "We have plenty of weapons already to hit them with."

The two U.S. nuclear weapons laboratories, Los Alamos and Lawrence Livermore national laboratories, have spent two years looking into the problems created by mobile missiles, according to a former Los Alamos official. "This is the first time the military services have been asked to study it," he said.

He added that a variety of approaches are being considered, including "electronic kill," a nuclear warhead that emits enormous amounts of electromagnetic pulse over a wide area to disable the electronics of a mobile ICBM launcher.

Although the recent emphasis of Energy Department nuclear weapons research has been on third-generation laser and particle-beam systems related to President Reagan's Strategic Defense Initiative (SDI), Pentagon and DOE officials told the House subcommittee earlier this year that new warheads are needed, not only because of mobile missiles but also because of the superhardened shelters being built for Soviet leaders and their military command and control systems.

The Air Force has ordered the Energy Department to study a "Hard Target Kill Weapon," according to the testimony. Wagner told the House members that "preliminary target analysis in the Pacific and Europe has identified more than deleted potential targets which are well hardened or underground."

He said concepts being investigated "include earth-penetrating weapons and high accuracy delivery systems." Los Alamos developed an earth-penetrating warhead for the Pershing II missile but it was never produced because its yield was too low and the missile could not reach Moscow from its bases in West Germany.

The Reagan administration has pointed to the need to develop new nuclear weapons as its major reason for refusing to negotiate a ban on underground nuclear testing or to join a Soviet moratorium on such tests.

The House hearings also confirmed earlier reports that the Army has dropped its plans to produce a new, small atomic demolition munition. The weapon system, which was to replace a 30-year-old system being withdrawn from Western Europe, had been publicly criticized. The Army sent a request to DOE to begin study of such a weapon, but shortly thereafter the Army canceled its request and indicated no future interest in developing a new atomic demolition munition, according to testimony.