Pakistani Air Force jets shot down one of four Afghan warplanes that flew several miles into Pakistan today, Pakistani officials said.

Local officials in Pakistan's Kurram district said two U.S.-built F16 fighters shot down one jet, which crashed inside Pakistan, and damaged a second, which flew back into Afghanistan.

The incident marked the biggest and most serious aerial clash between the two countries in the 6 1/2 years since the Soviet Union invaded Afghanistan and began fighting Moslem guerrillas who receive help from the United States and Pakistan. It was the first time an Afghan jet was brought down inside Pakistan.

The incident underlined Pakistan's difficult position in supporting the Afghan guerrillas, or mujaheddin -- a policy that many Pakistanis believe intensifies the danger of their becoming more directly embroiled in the conflict. Pakistanis as well as Afghan refugees have been killed in Afghan attacks on Pakistani border villages.

The four Afghan jets swept across the border shortly after dawn this morning and flew over the rugged mountains of the Kurram Agency, or administrative district, a "parrot's beak" of Pakistani territory jutting into Afghanistan that serves as a staging area for the mujaheddin, the officials said. While the government communique released in Islamabad did not specify how the planes were attacked, the officials in the Kurram Agency said that the two F16s fired air-to-air missiles, hitting two of the intruders. Pakistani F16s reportedly are equipped with the most sophisticated version of the U.S. Sidewinder missile.

The Kurram officials, in a telephone interview, said one jet crashed near the town of Parachinar, almost 10 miles inside Pakistan, and that a parachute had been found in the area. Pakistani troops began a search for the pilot, but government spokesman M. Yunis Sethi said late tonight that no trace of him had yet been found.

Some reports from Kurram officials said the Afghan planes were Soviet-built MiG21s, but Sethi said there was no confirmation of the type of plane involved. There was no immediate comment on the incident from Afghanistan.

MiG21s normally are flown by Afghan pilots, military analysts have said previously. The later and more advanced MiG warplanes based in Afghanistan, such as the MiG27, are believed to be manned by Soviet pilots. Today's combat came as Afghan and Pakistani diplomats in Geneva pursued the stalled, United Nations-sponsored talks aimed at an eventual political solution in Afghanistan. The two sides agreed yesterday to extend the talks. Pakistani military analysts have said that increased Afghan incursions along the border often coincide with the Geneva sessions -- an attempt, they say, to remind Pakistan of its vulnerabilty and increase pressure for Pakistani concessions in the talks.

Pakistan has protested numerous airspace violations, plus cross-border bombing and shelling by Afghan forces during six weeks of Soviet-Afghan government offensives in Afghanistan's Paktia Province. Pakistani officials say more than 30 persons have been killed in cross-border attacks in the past two weeks.

The protests have been low key. On almost a daily basis, small articles have appeared in Pakistani newspapers, noting that a senior Afghan diplomat in Islamabad had been summoned to the Foreign Ministry to receive a formal notice that the moral responsibility for the unspecified "serious consequences" of border incursions would rest with Kabul.

Pakistani observers have suggested that Islamabad does not want to emphasize the dangers of its backing the mujaheddin, a policy that is challenged by the leftist opposition and by many in the hard-pressed border areas burdened with masses of Afghan refugees.

"Air violations are running at about two and a half times what they were last year, but we are only protesting when there are casualties," an analyst close to the government said early this month.

Pakistan has also refused comment on reports that it shot down one Afghan jet in January and one in April, when they intruded into Pakistan. Those aircraft reportedly crashed in Afghanistan.

Having played down the border problem in the past, the Pakistani government today appeared set to change its tactics with the wreckage of an Afghan jet as proof of what the government calls "the blatant violations." Today's government announcement was made with unusual speed and spokesman Sethi said reporters would be flown to inspect the crash site.

This week, Soviet and Afghan forces reportedly intensified their attacks on villages, guerrilla bases and supply trails between the Afghan town of Jaji and the Pakistani border.