In the most serious recent incident in the war of nerves between Syria and Israel, Syrian jets fired air-to-air missiles today at Israeli aircraft that were on a reconnaissance mission over eastern Lebanon, the Israeli military command announced.

The Syrian missiles missed their targets and the Israeli planes, which did not fire back, returned safely to their base after the morning incident, according to the announcement. It added that the Israeli Air Force "is continuing its flights as usual."

In Beirut, police and radio reports gave another version of what could have been the same incident. They said a pair of Israeli jets encountered two Syrian warplanes 44 miles northeast of Beirut and parted quickly without a shot, Washington Post correspondent Herbert H. Denton reported.

No other details of today's confrontation were made public, but the military command also disclosed that on Monday two SA7 missiles--small, surface-to-air missiles carried by individual soldiers--were fired at an Israeli helicopter in Lebanon but did no damage.

Beirut radio reported that the Syrians shot down an unmanned Israeli drone aircraft about 25 miles southeast of Beirut today. The Israelis denied the report and said there were no other incidents reported along the line that separates the Israeli and Syrian armies in eastern Lebanon.

Syria gave no official version of today's reported events, with Syrian newspapers simply quoting accounts from Beirut.

Today's incident was the first confrontation between Israeli and Syrian aircraft since last Aug. 31, when Israeli jets shot down a Syrian MiG25 that was on a reconnaissance mission near Beirut. The last previous action of any kind involving aircraft over Lebanon occurred on Oct. 4, when Israeli jets destroyed a Syrian SA9 mobile surface-to-air missile battery that the Syrians had moved across the border into Lebanon.

By tonight, there were no reported countermeasures by Israel against the Syrians because of this morning's incident. The military command, however, issued a statement tonight accusing Syria of "a clear violation of the cease-fire agreement between Israel and Syria" and said the firing on the reconnaissance jets was "the most serious in the chain of increasingly serious Syrian violations" of the cease-fire.

The statement said the Syrian "provocation" was an attempt to nullify Israel's new troop withdrawal agreement with Lebanon and added that Israel "is closely following the Syrian trends and moves with the hope and desire of preventing violent clashes."

Since Israel signed a troop withdrawal agreement with Lebanon on May 17, Israeli officials have stressed they have no desire for a military conflict with Syria, appearing reluctant to risk the diplomatic gains from the Lebanon accord. The Israelis have played down the prospect of renewed fighting and characterized the reinforcement of Syrian forces in Lebanon's Bekaa Valley as largely a political move designed to subvert the Israeli-Lebanese agreement.

The Israeli Air Force has controlled the skies over Lebanon since the beginning of last summer's war and is believed to have flown constant reconnaissance missions over the country without challenge for months. In air-to-air combat during the first days of the war, according to Israeli figures, the Syrians lost at least 86 Soviet-made planes compared to none for the Israelis.

Israel acknowledges the loss of two planes from ground fire during the war.

Since then, however, the Soviet Union has resupplied the Syrian Air Force with front-line aircraft and installed new surface-to-air missile batteries in Syria, including long-range SA5 missiles capable of hitting targets over Lebanon or Israel.

Syria has rejected the Israeli-Lebanese troop withdrawal agreement and said it will not withdraw its forces or the remaining Palestine Liberation Organization guerrillas from their positions in eastern and northern Lebanon. But Israel has made its withdrawal contingent on a simultaneous pullout by the Syrians and the PLO.

According to the latest figures from the Israeli Army, the Syrian force in Lebanon and along the Lebanese-Syrian border totals about 50,000 soldiers, 1,200 tanks, 500 other armored vehicles and 400 heavy guns.

Israel has thinned out its military presence in Lebanon and is now said to have fewer than 20,000 troops there.

In Washington, a Defense Department official who asked not to be named said Syria recently sent an additional 1,000 troops into the Bekaa Valley. The official said the convoy, unlike a similarly sized influx about two weeks ago, did not appear to include Palestinian fighters.