An Israeli pilotless reconnaisance aircraft was shot down today by a Syrian surface-to-air missile while flying over the Bekaa Valley northeast of Beirut, the Israeli Army Command confirmed.

It was the first Israeli-confirmed downing of an Israeli aircraft over Lebanon since the Syrians moved Soviet-made antiaircraft missiles into Lebanon two weeks ago, and appeared to reflect a rejection by Syria of Israel's demand that any solution to the resulting crisis include freedom of movement in Lebanese skies for Israeli reconnaisance aircraft.

Israel has maintained unquestioned air superiority in Lebanese airspace since Syria sent peacekeeping troops there five years ago, shooting down 13 Syrian Mig fighters in the last 18 months without losing one piloted aircraft.The use of radio-controlled drones appeared to underscore the threat of the 14 surface-to-air missile batteries that Israel says have been deployed inside Lebanon or just over the Syrian side of the border.

Israeli officials said they had no information on which side of the border the missile downing the plane was fired from. When missiles were fired at an Israeli jet earlier this week, Israel said they came from Syrian territory.

[Dozens of persons in the village of Chtoura in eastern Lebanon said that they saw the missiles fired from there both at the drone and at a high-flying Israeli reconnaisance plane later in the day, Washington Post Correspondent Jonathan C. Randal reported from Chtoura.]

Israeli Prime Minister Menachem Begin told reporters outside his office that the downed drone proves that the Syrian missiles represent a "grave danger" to Israel's security. The Israeli Army Chief of Staff, Lt. Gen. Rafael Eitan, later said in an interview on Israeli television that Israel wil continue its overflights of Lebanon because it is necessary for national security.

The downing of the Israeli drone occurred as U.S. special envoy Philip C. Habib conferred in Damascus with Syrian officials in a shuttle diplomacy effort to defuse the crisis, which began to escalate April 28 when Israeli aircraft shot down two Syrian helicopters that had been used in attacks on Christian militias on Mount Sanin overlooking the Christian city of Zahle.

Habib returned here this afternoon to continue discussions with Begin and other Israeli officials. Tonight, after meeting with Begin, Habib refused to talk with reporters. Aides to the prime minister said only that the discussions would continue.

The Army Command refused to identify the type of the downed drone, but the most commonly used pilotless surveillance aircraft in the Israeli air force is a locally produced version of the U.S.-built Fire-B remote-piloted vehicle.

The Israeli model, which can be guided either by ground radio signal or preprogrammed with a flight plan, transmits television images as well as recording ground positions on film.

In October 1979, an Israeli drone flying over Lebanon and Syria was shot down by a Syrian Mig fighter.

On Tuesday, Syria claimed to have downed another Israeli drone, although the Israelis denied it, saying that pilots overflying the Bekaa Valley saw a missle explode harmlessly in the air. Wreckage of the aircraft displayed on Syrian television may have been from the 1979 downing, officials said.

Today's downing raised again questions of whether an Israeli air strike against the Syrian missile batteries would result in Israeli losses. Israeli electronics countermeasures used in deflecting Egyptian and Syrian SA2 missiles during the 1973 war are considered to be no longer adequate against later versions of the missile.The SA6 missile Syria now possesses has been upgraded with improved terminal guidance radar carrying a wider range of frequencies, according to Jane's Annual Arms Handbook.

Military analysts at London's Institute of Strategic Studies have suggested that the diversity of the Syrian missile deployment -- including batteries of SA2s, SA3s, and SA9s -- could make it difficult for Israeli jets to lock onto so many homing frequencies and deflect the missiles. The Israeli jets might have to approach their targets at low altitudes, which would make them vulnerable to conventional antiaircraft weaponry, analysts said.

The Israeli Foreign Ministry today declined to react to the drone downing, noting that "many Syrian planes" have been shot down without altering the political situation with Syria.

A Foreign Ministry source said today's incident could be interpreted as an escalation by Syria, or it could be viewed as "a kind of satisfaction for [Syrian President] Hafez Assad and an opportunity for him to pull back."