Reports that India might launch an attack on Pakistan's nuclear facilities are regarded as "a serious threat" that has prompted "appropriate defensive measures," Pakistani Foreign Minister Sahabzada Yaqub-Khan said yesterday.

The Pakistani official, who has been meeting U.S. officials on a visit to Washington, said any such Indian attack would be "naked aggression" and would leave Pakistan with "no alternative but to retaliate," with the likelihood of touching off a broad conflict.

Yaqub-Khan, in a meeting with journalists, was reacting to press reports emanating from a U.S. intelligence briefing on Capitol Hill in mid-September that Indian Prime Minister Indira Gandhi is being urged by military advisers to attack Pakistan's developing nuclear facilities. At the time, State Department officials discounted the reports as "alarmist" even though they conceded that some intelligence information about a possible Indian attack had been received.

According to Indian state radio, Gandhi said yesterday in an address to Indian army commanders that "Pakistan's nuclear program has brought about a qualitative change in our security environment." She was quoted as criticizing continued U.S. military aid to Pakistan in the light of "evidence compiled by Americans themselves about Pakistan's nuclear program."

The same state radio account quoted Indian Defense Minister S.B. Chavan as emphasizing "the imperative need for vigilance and full defense preparedness in view of the changes in the security environment." Chavan reportedly referred specifically to "feverish efforts" by Pakistan at military modernization and its "ceaseless quest for acquiring nuclear capability."

An Indian nuclear explosion in 1974 alarmed the world and spurred Pakistani efforts to match it. Pakistan has officially denied, as Yaqub-Khan did yesterday, that its extensive nuclear program is intended for military purposes. But the Carter administration temporarily suspended U.S. aid because of evidence that Pakistan was trying to acquire atomic weapons, and the Reagan administration continues to be concerned about the Pakistani nuclear program.

Yaqub-Khan did not say what defensive measures had been taken in light of the Indian threats, but U.S. sources said there are reports that some Pakistani nuclear facilities have been moved underground.

According to Yaqub-Khan, there has been a "downturn" recently in Pakistani relations with India. He expressed concern about increasing clashes between Pakistani and Indian mountain troops along the disputed border in Kashmir but said "we will not be provoked."

A State Department spokesman said reports of these clashes are "obviously cause for concern" and that "the United States firmly believes any problems existing between India and Pakistan should be settled though negotiations and not through force of arms." The spokesman noted that the skirmishes are reported to be taking place in an inhospitable mountain area and said, "The immediate situation is likely to be resolved by weather."

Yaqub-Khan said Pakistani relations with the Soviet Union and the Soviet-backed regime in Afghanistan have also suffered a "downturn" in the past two or three months. He cited "sneak raids" by Soviet-supplied Afghan warplanes against targets in Pakistan near the Afghan-Pakistani border, and stiff Soviet diplomatic charges that Pakistan is aiding anti-government rebel forces in Afghanistan.