Israeli warplanes shot down a Syrian Mig 25 fighter that intercepted them today during a photo reconnisance mission over Lebanon, the Israeli military command said.

It was the 15th Syrian Mig downed by Israeli fighters since April 1979 and the first Syrian Israeli encounter in Lebanese skies since an agreement by Israel and the Palestine Liberation Organization last Friday to cease all hostile military actions across the Israeli Lebanese border.

In another development, Palestinian guerrillas ambushed an Israeli public bus tonight, wounding at least four with submachine-gun fire, Israeli police said.

Authorities said the Israeli bus was attached along a narrow road off the main Jersalem-Tel Aviv highway approaching the Maale Hachamisha kibbutz, which maintains a tourist guest house. The police said the gunmen ran into a forest after the attack and were being sought by Israeli border police.

The kibbutz is located near the Arab village of Abu Gosh, close to the pre-1967 border between Israel and the West Bank.

[PLO chief of operations Abu Jihad was quoted in a Beirut newspaper on Sunday as saying, "Our operations inside the occupied territories are one of our people's rights" and that such actions were not covered by the cease-fire agreement, The Associated Press reported.]

An Israeli defense forces spokesman said the downing of the Syrian jet followed a brief dogfight during a "routine" aerial surveillance mission. An Israeli Army communique said that under the terms of the cease-fire agreement, the Israeli Air Force is permitted to continue reconnaissance flights in Lebanese airspace.

Israeli military sources said the Syrian fighters engaged the Israeli aircraft during a "nonhostile" operation and that the Israeli pilots responded in self-defense. All of the Israeli planes returned to their bases, the Army command said.

A Syrian military spokesman in Damascus confirmed that one of its planes had been hit, but said an Israeli aircraft was also downed. Reuter reported from Beirut. The Syrian spokesman also said the plane was a Mig 23, not the more sophisticated Mig 25 as reported by the Israelis.

[The Syrian pilot parachuted to safety and was picked up in a hilly area about 30 miles northeast of Beirut, Lebanese sources quoted by Reuter said.]

The clash is likely to intensify the debate over whether Israeli overflights, which Prime Minister Menachem Begin insists are essential to maintain the security of Israel's northern border, are covered by the cease-fire.

The Israeli communique said: "As may be recalled, Syria reiterated yesterday that if Israeli planes continue flying within Lebanese airspace, Syria will intercept the planes and shoot them down." The communique added that Israel is "authorized" by the cease-fire to conduct reconnaissance flights and that this position has been endorsed by the U.S. State Department spokesman and by U.S. special envoy Philip C. Habib.

[The State Department, through spokesman Dean Fishcer, has said that reconnaissance overflights could not be considered hostile attacks, but the department has refused to draw the conclusion publicly that overflights are permitted under the cease-fire agreement.]

[Reacting to today's dogfight, Fischer called it "disturbing" and added that "we hope that there will be no repetition." But he declined to comment about who was held responsible or to be more specific.]

Last Friday's announcement of the cease-fire agreement, the details of which have not been made public, did not make clear how, if at all, the agreement extended to Syria, which had maintained a large military force in Lebanon.

PLO spokesman Mahmoud Labadi, referring to the overflights as "an act of arrogance, whether the American administration wants to admit it or not," said the PLO regards Israeli aerial surveillance as "blatant violation of the cease-fire. An official Syrian statement made the same charge, warning that the Israeli planes would be challenged.