China has provided Pakistan with sensitive information about the design of nuclear bombs that could assist it significantly in its drive to develop a nuclear weapons capability, according to U.S. intelligence sources.
By confirming for Pakistan that a particular weapon design would work, the sources said, the Chinese may have made it possible for Pakistan to proceed with its effort to build atomic bombs without staging an early nuclear test that would bring a cutoff of American military aid.
While a variety of industrial nations over the years have wittingly or unwittingly provided technological assistance to developing countries that helped them produce materials such as plutonium, which can be used in nuclear bombs, no other weapons state in recent history has knowingly provided design aid of this kind.
Thus far, only a nation with the scientific and engineering sophistication of Israel is believed to have had the confidence to build a nuclear arsenal without testing an initial weapon. Pakistan is not viewed as having that level of technological knowledge.
Since China has staged a variety of nuclear weapons tests over the years, its advice on this subject to its longtime ally clearly would be of considerable value.
Secretary of State George P. Shultz plans to raise U.S. concern over China's assistance to the Pakistani nuclear program when he travels to Peking next week, according to sources.
Shultz is prepared to make clear to the Chinese that progress cannot be made toward a bilateral agreement of nuclear cooperation that would let U.S. companies sell civilian atomic power plants to China unless Peking provides assurances that it will not assist other countries in developing nuclear weapons.
"We simply have got to get this problem cleared up if China wants to buy power plants or components from companies like Westinghouse," an administration source said.
The classified reports of Chinese assistance to Pakistan, which originated with the British several months ago, have been the subject of considerable concern and debate within the Reagan administration.
While some analysts, including the British, did not attach too much importance to the reports initially, the consensus now appears to be that China provided significant information to Pakistan.
U.S. diplomatic representations have been made to Peking in the past year on this and other Chinese nuclear dealings, such as some exports to Argentina. To date, sources said, the Chinese response has not been totally satisfactory.
But unlike previous Chinese nuclear deals--which Reagan administration officials have tended to explain away by suggesting that Peking is not yet a sophisticated exporter, and may have been duped--there cannot have been any doubt about why Pakistan was seeking weapons design information, the sources said.
Since the mid-1970s Pakistan has had a program clearly aimed at developing nuclear weapons. While U.S. efforts to frustrate that program by blocking sale of components and technology have delayed Pakistan at least a couple of years, intelligence sources say there has been no decrease in Pakistan's efforts.
State Department officials have been saying recently that Pakistan appeared to be making preparations to stage a nuclear test in 1979, 1980 and early 1981, but now appears to be backing away from such preparations.
These officials suggested that the shift demonstrated that the improved U.S.-Pakistani security relationship was having a positive impact in slowing Pakistan's nuclear program, but other sources said that such a shift might as easily be attributed to a diminished Pakistani need for a test explosion.
Diplomatic sources said that if U.S.-Chinese talks on a bilateral atomic agreement are to progress, a pledge or commitment by Peking not to help other countries with nuclear weapons programs is required.
Such a pledge would be used by the administration to persuade Congress to agree to sale of atomic power plants or components to China, even though Peking has made clear it has no intention either of ratifying the nuclear non-proliferation treaty or accepting international safeguards on all its nuclear facilities.
While the latter position has been viewed as a major obstacle to an agreement, administration sources say they believe that, since China already is a nuclear weapons state, some arrangement could be struck where specific power plants sold by American firms would be open to inspection even though Peking's weapons production facilities remained closed.