Indian Prime Minister Rajiv Gandhi and Pakistani President Mohammed Zia ul-Haq are reported to have agreed here today to begin "technical talks" aimed at averting a nuclear weapons race in South Asia.

After their morning meeting, the Asian leaders met separately with President Reagan in his suite in the Waldorf-Astoria Hotel, where Reagan expressed his concern about the prospect of an "intensifying nuclear competition" between India and Pakistan, according to a senior administration official who participated, and who later briefed reporters.

The U.S. official said that, despite the day's discussions, "it's a little early to draw conclusions" about the willingness of India and Pakistan to work together in this field. The official said that meetings such as the Gandhi-Zia session today were important in dispelling mistrust and working toward a more cooperative relationship.

As if to demonstrate the distrust between the two nations in this area, there was a dispute here tonight about what, if anything, had been decided in the Gandhi-Zia meeting.

A Pakistani spokesman said the two had agreed on the desirability of initiating "a process of technical discussions between the two countries on nuclear nonproliferation issues." But the chief Indian spokesman, Sharada Prasad, said he could authoritatively say that no agreement on technical talks had been reached.

Other Indian sources said Gandhi and Zia appeared to believe that further discussions on the nuclear issue might help but that no definite time or place for any such talks had been set.

The importance in practice of technical discussions that are expected to flow from today's agreement is difficult to predict, according to U.S. experts. But the agreement, if it has been reached, could be at least a start in reversing the deepening mutual suspicion between the two countries, each of which has an advanced nuclear program that has been the subject of widespread international speculation and concern.

A Pakistani official said the technical talks are expected to be arranged in detail during a planned trip to India in the coming weeks by Munir Ahmed Khan, chairman of the Pakistan Atomic Energy Commission.

An Indian official was less positive. He said that there had been no agreement today about precisely how to begin such discussions.

The Pakistani official said Gandhi and Zia agreed today that they must find some way to allay the mounting concerns about their nuclear programs. Each of the leaders assured the other of the peaceful purposes of his country's nuclear program, the official said.

Beyond their discussion of the nuclear issue, Gandhi and Zia authorized immediate discussions in New York of their senior Foreign Office civil servants on ways to staunch illegal traffic across their porous border, and ways to increase the flow of trade between them.

The two officials, Indian Romesh Bhandari and Pakistani Niaz Niak, are expected to explore instructions to officials of the two governments to deal with the border and trade issues, according to an Indian official.

The nuclear issue has been one of intense concern in South Asia since India exploded a nuclear device in 1974.

Pakistan, which had already begun work on its own nuclear program, is believed to have redoubled its efforts following the Indian blast.

In recent months there has been a growing belief in India that Pakistan's nuclear program, some of which has employed clandestine means of supply that have been actively opposed by the United States and other nations, had reached the point of weapons capability.

Reagan, in an interview with J.N. Parimoo, Washington correspondent of The Times of India, said Monday that "we have no evidence" that Pakistan has the bomb.

Indian officials, however, said they do not share the assessment that Pakistan has not reached nuclear weapons capability.

Some Indian estimates suggest that Pakistan has acquired enough nuclear material to make five explosive devices. Pakistan, however, denies having nuclear weapons or planning to acquire them.

Zia, speaking today to the General Assembly in its 40th anniversary session, said, "I take this opportunity to reaffirm Pakistan's policy of developing nuclear energy for peaceful purposes only and its irrevocable commitment not to acquire nuclear weapons or nuclear explosive devices. Pakistan has neither the capability nor the desire to develop nuclear weapons."

Zia, in an interview Monday with The Washington Post, expressed apprehension that India may already have manufactured nuclear devices "for military use." Zia also expressed concern about India's new fast breeder nuclear reactor, saying it could produce "unchecked" and "unsafeguarded" nuclear material.

Responding to rising concern on both sides, Reagan last month sent Undersecretary of State Michael H. Armacost and Donald R. Fortier of the National Security Council staff to New Delhi and Islamabad, primarily to discuss nuclear matters. The two officials were unable to report any specific progress as a result of their talks.

Reagan, in his Times of India interview, said both nuclear weapons proliferation and the miniaturization of nuclear weapons, which could bring them within the reach of terrorists, are "a grave threat" to international peace and stability.

"We recognize that a country's sense of its security may lead it to look for a nuclear option," Reagan said, adding that nuclear weapons actually can add to insecurity.

"We hope that the countries of South Asia will set an example by forgoing nuclear weapons," he said.