India's military leaders have prepared a contingency plan for a preemptive strike against Pakistani nuclear facilities and proposed such an attack to Prime Minister Indira Gandhi earlier this year, according to U.S. intelligence sources.

Gandhi decided against carrying out an attack when she first heard the proposal nine months ago but did not foreclose the option of striking if Pakistan appeared on the verge of acquiring a nuclear weapons capability, the sources said.

K.R. Narayanan, India's ambassador to Washington, yesterday termed reports of the military contingency plan "a figment of the imagination."

"As far as I know, we have absolutely no intention of striking Pakistan to destroy its nuclear installations or for any other purpose," he said. "Our policy has been not to start any kind of conflict with Pakistan . . . ."

In New Delhi, Foreign Ministry spokesman Mani Shanker Aiyar repeated Narayanan's comment and said he would not comment further.

H.Y. Prasad, Gandhi's spokesman, said, "There is no substance in this at all. There is no proof . . . ."

The level of Indian concern about Pakistan's activities has risen sharply in the last few weeks and apparently is linked to completion of a clandestine plutonium reprocessing facility on the outskirts of Islamabad, according to U.S. sources.

Asked whether such concern has increased recently or whether India had received any information to cause increased concern, Prasad replied, "Naturally, Pakistan's nuclear program or any part of its nuclear program would cause concern." He declined to comment further.

While U.S. sources said no plutonium has been reprocessed at the so-called New Labs plant built next to the Pakistan Institute for Nuclear Science and Technology, Pakistan appears able to begin operating the facility at any time.

The Indian and U.S. governments are concerned that Pakistan intends to use New Labs to reprocess spent fuel diverted from the Kanupp atomic power station outside Karachi.

The International Atomic Energy Agency has been unable for 21 months to keep track of the amount of plutonium produced there because Pakistan no longer depends totally on verifiable quantities of imported fuel for the reactor.

U.S. sources said Pakistan has been operating Kanupp during that time at a reduced power level and could have produced 10 to 20 kilograms of weapons-grade plutonium, enough material for one to three Hiroshima-sized weapons.

The Indian contingency plan, according to sources, called for an air strike at the reprocessing plant and at a uranium enrichment plant being built nearby in the tiny village of Kahuta.

The enrichment plant, on which Pakistan began working clandestinely in the mid-1970s, represents a longer-range threat since intelligence experts believe it is unlikely this facility can produce weapons-grade uranium at least until the mid-1980s.

But unlike the reprocessing option, which under present circumstances probably could not provide Pakistan with material for more than a couple of nuclear bombs, the completed enrichment plant could produce a continuing supply of highly enriched uranium suitable for atomic weapons.

A major consideration in Gandhi's decision at least to defer a preemptive strike, sources said, was concern that India's nuclear facilities might be vulnerable to a retaliatory Pakistani air strike.

This concern presumably has increased with Pakistan's acquisition earlier this month of the first installment of U.S. F16s, the type of planes Israel used in bombing Iraq's atomic research reactor in June, 1981.

While Iraq's reactor was not yet in operation, India is operating various civilian atomic power plants and research reactors.

If even conventional bombs were dropped on one, radioactive materials could be spread over highly populated areas.

Pakistani President Mohammed Zia ul-Haq said in an interview at The Washington Post two weeks ago that his country "naturally has a concern" about an Indian military attack on its nuclear facilities, particularly in light of Israel's strike in Iraq.

"We have taken adequate security precautions," he said. While Zia did not elaborate, U.S. intelligence sources said the Kahuta plant is ringed with French-built Crotale surface-to-air missiles.

"But under the present environment, I don't think there is a necessity of India to be that hostile," Zia said.

"We are not in competition with India. We have not developed, are not capable of developing and have no intention of developing an atomic bomb."

While Zia acknowledged "there may be some concern" in India about Pakistan's nuclear program, he said the subject "never came up" when he met with Gandhi two months ago in New Delhi.

The two agreed then to open talks aimed at concluding a nonaggression treaty. A major consideration in Prime Minister Indira Gandhi's decision at least to defer a preemptive strike, sources said, was concern that India's nuclear facilities might be vulnerable to a retaliatory Pakistani air strike.

Some U.S. sources consider it unlikely that India would mount a preemptive strike against Pakistan while the bilateral talks between the two countries are continuing.

Zia said he told President Reagan categorically here that Pakistan is not attempting to acquire "a nuclear capability in the military field.

"Pakistan is trying to acquire in a very humble way nuclear technology for peaceful purposes," he said.

Although U.S. intelligence has known for several years that Pakistan was clandestinely building the New Labs reprocessing plant from a design prepared by the Belgian firm Belgonucleaire, Zia said at The Post:

"We have no reprocessing facility whatsoever. Pakistani scientists are experimenting with how to reprocess one ounce of plutonium as scientists. You cannot deny scientists the right to experiment."

U.S. analysts said New Labs' capacity is considerably greater. Munir Khan, head of Pakistan's nuclear program, has told Europeans that the plant would be large enough to reprocess the half-dozen kilograms of plutonium needed for a nuclear explosive device.

Zia conceded that Pakistan was building an enrichment facility but described it as "a very humble, modest program."

U.S. sources expressed skepticism about Zia's description of the Kahuta plant, being constructed from stolen plans of the huge Urenco enrichment facility in the Netherlands, as humble.

The plant is designed to use 10,000 ultracentrifuges, manufactured to very fine tolerances from special steel alloys, to spin uranium hexafluoride gas at speeds of as many as 100,000 revolutions per minute.

"We have devoted a very meager percentage of our budget to this," Zia said at The Post.

Pakistani Finance Minister Ghulam Ishaq Khan later insisted that the government's "total expenditure on the nuclear program" totals only "$50 million to $55 million annually" and said that sum includes uranium exploration activities and production of radioactive isotopes for medical and other purposes.

U.S. intelligence sources pointed out, however, that while the enrichment plant and reprocessing facility are supervised by Pakistan's Atomic Energy Commission, purchasing of equipment and construction of the nuclear plants is handled by the military.

Since the mid-1970s, Pakistani purchasing agents, operating under various guises, have been attempting to purchase parts for both facilities under false pretenses in Europe, Japan and the United States, according to U.S. intelligence sources.

The Reagan administration has privately told congressional leaders that this effort continues unabated.

The United States, sources said, continues to make diplomatic representations to other countries virtually every week urging them not to sell Pakistan items that could aid its nuclear program.