The United States caught Pakistan too late to keep it from attaining the capability to make nuclear weapons within two to five years in one of the most unstable areas of the world, a State Department official told Congress yesterday.

Assistant Secretary of State Thomas R. Pickering said Pakistan managed to acquire enough of the special equipment for producing weapon-grade uranium by making "end runs" around international export restrictions before it was discovered and the flow stopped.

The discovery means it will take Pakistan longer to get the weapons, he said.

"We believe we have the capacity to slow down that kind of activity. But no one is willing to say. . . we have the ability to stop it," Pickering told the Senate Covernmental Affairs subcommittee on nuclear proliferation.

With the equipment it has been able to get, Pickering estimated, Pakistan will be able to produce enough highly enriched, weapon-grade uranium to build nuclear weapons in two to five years.

With the failure of efforts to stop Pakistan from getting the equipment, Pickering said the United States has switched to diplomacy to persuade Pakistan to halt its military nuclear program.

These efforts have produced no positive results, the State Department official said.

He denied published reports that the United States had offered to sell Pakistan fighter planes and help with its nuclear power program if it would give up trying to develop nuclear weapons.

Intelligence that Pakistan is moving into the nuclear weapon field - which Pakistan denied - clearly upset the State Department. Last month it cut off all military and development aid to Pakistan because of its nuclear weapons push.

"We are concerned that Pakistan's program is not peaceful but related to an effort to develop a nuclear explosive capacity," said Pickering.

"Pakistan's efforts have serious implications for stability in the region and more broadly," he continued.

That region is one of the most volatile in the world. Pakistan is considered by analysts here to have potential internal problems; Iran is in the midst of a revolution that overthrew the shah, and tribesmen are rebelling throughout Afghanistan.

While subcommittee chairman John Glenn (D-Ohio) suggested that India's explosion of a nuclear device in 1974 triggered the Pakistani action, Pickering said the general instability of the area also contributed to it.

Pickering refused in open session to discuss reports that Libya's Col. Muammar Qaddafi - who is known to want a Muslim nation to have a nuclear bomb to match the one Israel is believed to have - was financing Pakistan's nuclear weapons program.

This was one of about two dozen questions - most referring to intelligence gathering or diplomatic activities - that Pickering said should be taken up in closed sessions.

Pickering said the government has no indications that other nations are trying to develop nuclear weapon capability the way Pakistan did, but other countries are being watched closely "as potential proliferators of the future."

Glenn and Pickering agreed that Pakistan had developed a method to get around export restrictions on nuclear-enriched equipment by buying bits and pieces of the equipment from suppliers around the world, claiming they were to be used in such peaceful industries as testiles.

Meanwhile, the State Department said Tuesday that several European nations have agreed to work with it in preventing Pakistan from buying more such equipment. The Swiss government also announced that it is investigating sales by several Swiss companies to Pakistan, and American officials said they are seeking cooperation from Britain, France, West Germany and Japan.