Israeli warplanes flying over Lebanon's Bekaa Valley shot down two Syrian interceptors that rose to meet them today, sending both crashing in flames into the rugged mountains northeast of Beirut.

The aerial encounter was the first since April 21 between Israel's U.S.-made jets and the Soviet-supplied Syrian Air Force. Both Israel and Syria regard Lebanon's skies as part of their defense perimeter. Today's air action seems likely to renew tension in Lebanon after two weeks of what looked like improved chances for calm.

The Israeli planes were on a routine mission, presumably reconnaissance, according to Israeli spokesmen cited in dispatches from Tel Aviv that reported the downing of the two Migs. Lebanese officials said they were flying near Syria's SA6 antiaircraft missile sites in the eastern Lebanese valley, but that no missile was seen being fired.

The Syrian decision to intercept Israeli jets--which on most days fly unmolested over Lebanon--was seen as a demonstration of President Hafez Assad's refusal to abandon his self-assigned role as protector of Lebanese air space despite the Israeli Air Force's evident superiority.

Western diplomatic sources in Damascus have reported signs of increased Syrian resolve to confront Israeli actions in Lebanon, particularly any large-scale attack on Palestinian guerrillas or the invasion of southern Lebanon repeatedly threatened by Israeli leaders. Part of the resolve stems from Syria's feeling that Soviet backing is likely to be firmer than in the past, these informants said, although the extent of Moscow's commitment to help has not been spelled out.

Dispatch of Migs to intercept the Israeli planes remains a largely symbolic gesture, however, because Israel's Air Force has scored a steady string of kills over the Syrian jets. In the April 21 clash, for example, Syria lost two planes and Israel none. Israeli authorities said all their planes returned to base unharmed today as well.

Washington Post correspondent William Claiborne reported from Jerusalem that Israeli military officials said that since the May 31, 1974, Syrian-Israeli disengagement, 19 Syrian aircraft have been downed by Israeli jets.

A spokesman for Lebanon's Christian Phalange militia said two Syrian pilots ejected and parachuted to safety, one near the village of Qulayat and the other near the village of Deir Choueir. Both little communities lie in the rough Mount Lebanon area about 15 miles northeast of Beirut.

Both pilots were hospitalized and were scheduled to be turned over to Syrian authorities, he said. One was said to be suffering from broken ribs, the other from bruises.

Wreckage from the two Syrian warplanes was found in separate sites near the two villages but there was no immediate identification of which Mig models were involved in the clash.

In previous attempts to confront Israeli planes, Syria has sent up MiG21s and MiG25s, neither of which has proved a match for Israel's F15s and F16s with their sophisticated electronic missile-guidance systems and radar.

A Syrian military spokesman acknowledged in Damascus today that two Syrian planes were "hit," but did not say whether they were shot down.

A number of Israeli jets were spotted over Lebanese territory, particularly in the Bekaa Valley, shortly after the brief battle. It was unclear whether they were making new reconnaissance runs or driving home the point that the Israeli leadership intends to continue flights in Lebanese air space.

Lebanon itself has no practical way to defend against Israeli overflights. Its Air Force has 11 British-made Hunters, subsonic craft of another generation, and nine French-made Mirages, only a few of which are in good repair.

The Syrian SA6 missile batteries were put into Lebanon a year ago in response to Israel's shooting down of a pair of Syrian helicopters. Although they caused a crisis at the time, little has been said about them in recent months.