Conservative and liberal members of the House Foreign Affairs Committee joined in putting Reagan administration officials on the defensive yesterday, criticizing the newly signed nuclear cooperation agreement with China as lacking guarantees against the spread of nuclear weapons.

The unlikely allies praised Rep. Edward J. Markey (D-Mass.) for his testimony that some of the pact's language is "fatally flawed . . . vague, elastic and unverifiable." Markey urged the committee to return the pact with a recommendation that the White House resubmit it to Congress as an "exceptional" agreement that does not satisfy U.S. law.

Richard T. Kennedy, the U.S. ambassador-at-large who negotiated the pact, said it meets all provisions of U.S. law and the Nuclear Nonproliferation Treaty. Kenneth L. Adelman, director of the U.S. Arms Control and Disarmament Agency, testified that the pact with the Chinese "helps ensure that they are part of the nonproliferation solution, rather than part of the problem."

Markey's objections, Adelman said, "are already fixed and have been fixed for a long while."

The agreement, signed last week during Chinese President Li Xiannian's visit to Washington, outlines conditions under which the U.S. nuclear industry may bid for a share of China's nuclear power plant construction. At least $6 billion in foreign business is expected to be involved.

The pact was initialed last year but was not submitted to Congress after intelligence reports that Chinese technicians had been spotted at the suspected site of a Pakistani effort to develop nuclear weapons. Administration negotiators sought further assurances from China and have said that China is now committed to halt nuclear proliferation.

Markey focused on a section of the pact that provides for consultations on future Chinese desires to reprocess spent U.S. nuclear fuel -- a procedure that can yield material usable in weapons -- and on a section providing for talks to arrange compliance inspections.

"A promise to talk in diplomatic channels is not a guarantee of anything," he said. "The language . . . is an embarrassment."

Rep. Gerald B. Solomon (R-N.Y.), expressing surprise that he could agree with the liberal Markey, endorsed Markey's concerns, as did Rep. Robert K. Dornan (R-Calif.).

Rep. Henry J. Hyde (R-Ill.), normally a staunch administration supporter, asked Kennedy why stronger language had not been sought.

"Agreeing to agree is what it says, but it also says that without that agreement nothing will happen," Kennedy said, adding that stronger terms "would have been taken as an affront to the national sovereignty" of China.

Markey offered to provide "boilerplate language" from other nuclear cooperation agreements that he said would make the pact acceptable, if the committee should choose to amend it. Such a change would require renegotiation with the Chinese and would be rejected by the administration, Kennedy said during a break in the proceedings.

Energy Secretary John S. Herrington stressed that China regards development of a nuclear industry as "important to its energy security." He said the sales would not only ease China's demand for oil but would also help the U.S. nuclear industry, which has not sold a reactor domestically since 1978.

Paul Wolfowitz, assistant secretary of state for East Asian and Pacific affairs, testified that the pact would allow the United States to influence China's nuclear future.

"China is a nuclear weapons state and has been for 21 years without this agreement. China is a significant supplier of nuclear materials to third countries and will continue to be with or without this agreement," he said.

"There has been a clear change in Chinese policy" toward controlling weapons spread, he said, and the desire for continued access to U.S. nuclear technology is "a powerful incentive" for adherence to the pact.

Markey said doubtful members of Congress should obtain a classified briefing from the Nuclear Regulatory Commission on its concerns about the pact. "It is a very sobering experience," he said.