Is the People's Republic of China helping Pakistan to build a nuclear weapon? If not, why is it not prepared to give the United States stronger assurances on this score?
These are the questions being raised here as the US-Chinese agreement on commercial nuclear cooperation has run into trouble. President Reagan initialed the agreement during his recent visit to China and it was billed as a key achievement of his visit.
But as the agreement has gone through the process of interagency review before being signed by the President and submitted to Congress, new information appears to have come to light to challenge Peking's claim that it is not currently providing nuclear weapons assistance to Pakistan.
As a result, the commercial agreement is almost certain not to take effect this year. Even if signed by the President, it must be submitted to Congress where it must lie for 60 days of continuous session.
''Given the congressional timetable, that won't be possible,'' says a congressional aide. ''It will have to be picked up next year.''
The Chinese have given assurances not to transfer to third countries any nuclear material resulting from US reactors sold to China. That has been written into the cooperation agreement and is not the fundamental problem, say US officials. The contentious issue, they say, pertains to China's stand on nuclear nonproliferation in general.
The Chinese are said to be extremely irritated by the turn of events. Premier Zhao Ziyang pledged in a White House dinner in January that his country does not engage in nuclear proliferation or help other countries develop nuclear weapons. That pledge has been repeated in a statement adopted by the Chinese National People's Congress, its nominal parliament. It also has been reiterated by the Chinese Foreign Ministry.
But the United States has persisting suspicions about Pakistan's nuclear plans and China's role in them.
Sen. Alan Cranston (D) of California charged in a recent speech that Pakistan is building a nuclear device, and that it has conducted reprocessing of spent fuel from a nuclear reactor. That would be in violation of International Atomic Energy Agency (IAEA) safeguards.
The administration also has some doubts. According to congressional sources, US Deputy Assistant Secretary of State William Brown briefed legislative aides last week about continuing intelligence evidence that Pakistan may be developing a nuclear weapon with China's help. The evidence is not conclusive and the administration is pursuing the matter through diplomatic channels.
The Chinese are sensitive to the issue of providing guarantees because they feel it infringes on their national sovereignty, making it appear that they are knuckling under to outside pressure.
Over the weekend, a White House spokesman said that the US-China cooperation agreement would be submitted to Congress only after the administration is ''satisfied there is a mutual understanding between us and the Chinese.'' Political observers suggest that the Reagan administration does not wish to be caught in a Democratic campaign charge that it is weakening international efforts to stop the spread of nuclear weapons.
Pakistan has a power-generating reactor near Karachi which is under IEAE safeguards. But there was a period of about a year when the IEAE did not have confidence that material from the reactor was not being removed.
According to press reports, the Pakistanis may be working on uranium enrichment at a plant near Islamabad using centrifuge technology stolen from the Dutch. Inasmuch as China does not have a centrifuge enrichment plant, say some experts, it cannot be ruled out that China and Pakistan are cooperating for mutual benefit.
One unanswered question, nuclear experts say, is whether China transferred to Pakistan in 1983 the design of the device exploded in its fourth uranium bomb test, a move that would allow Pakistan to build a bomb without conducting a test.
While there appears to be no solid proof that China is providing nuclear weapons aid, supporters of nuclear nonproliferation are disturbed by the emergence of new suspicions.
''If it's true that Pakistan has reprocessed fuel, that means a violation of international safeguards and that would be an extremely grave step,'' comments Leonard Spector of the Carnegie Endownment for International Peace. ''We could be at the brink of a major development where US attitudes are concerned.
''What will the United States do in response? Will it stand by the international nonprofliferation regimen?''
The Westinghouse Electric Company and other American firms are said to be angered by the delay in the US-China agreement. The Chinese are holding bids on the construction of two nuclear power plants. French firms are expected to be the recipients of the large contracts, but American firms had hoped to provide related services.