An Army command communique said tonight that Israel "had no choice" but to destroy Syrian missiles positioned in Lebanon's Bekaa Valley in a massive air attack today, and the Israeli government blamed Syrian President Hafez Assad for provoking the attack.

Defense Minister Ariel Sharon said that the Israeli action means a "turning point" has been reached in the war in Lebanon and that the Syrian buildup is on the verge of collapse.

His assessment came as Israel mobilized reserve forces and deployed large numbers of troops in the Golan Heights in anticipation of the possible opening of another front in a rapidly escalating confrontation with Syria.

Sharon said that it is only a matter of time before Israel's invasion forces clear central Lebanon of Palestinian guerrillas and "forever" relieve northern Israeli settlements of the threat of PLO artillery attacks.

In Washington, Israeli Ambassador Moshe Arens said that, despite Israel's public promises that it did not want to engage Syrian forces, Syria had deployed additional forces of about 18,000 men into Lebanon on Tuesday and extended its areas of operation to the point where they impinged on the edges of Israeli operations. He also said that the Syrians had reinforced their 13 surface-to-air missile sites in the Bekaa by moving in an additional six.

Arens said that 17 of the 19 sites were destroyed and the other two were severely damaged.

He also said that in a meeting Tuesday with U.S. special envoy Philip C. Habib, Prime Minister Menachem Begin had asked Habib to go urgently to Damascus to repeat Israel's original assurances to Assad and to inform him that Israel was ready to assume that the new Syrian deployments were reversible. However, Habib was unable to undertake the Damascus trip until today, Arens said.

As if aware of the danger of pushing Assad into launching an attack on Israel from the eastern front, the Army command announced that it had deployed troops in the Golan Heights.

"We consider the Golan Heights to be a potential theater in addition to Lebanon, a spokesman for the Israeli Defense Forces said tonight.

The Israeli Army radio had said that reservists were being mobilized for a buildup in the Golan Heights large enough to resist a Syrian attack from that direction. U.N. observer forces in the Golan Heights said they had not seen any unusual Syrian troop movements there that would indicate an offensive footing.

As Israeli armored columns pushed north through central Lebanon toward the strategic Beirut-Damascus highway, the Army command announced that waves of Israeli fighter-bombers knocked out Syrian SA6 missile batteries in the Bekaa Valley, freeing Israeli ground forces to pursue Palestinian guerrilla units in central Lebanon.

Israeli jets downed 22 Syrian Mig21 and Mig23 warplanes during battles over the Bekaa Valley, Sharon said, dealing a crippling blow to Syria's ability to defend its forces in Lebanon.

The Army command said the decision to destroy the missile batteries--which were deployed in the Bekaa Valley in April 1981 after Israeli jets shot down two Syrian helicopters--was taken this morning in an emergency Cabinet meeting.

In a press conference in the northern Israel city of Safed after the Bekaa Valley air strikes, Sharon said that a "turning point" had been reached in the campaign in Lebanon because the missiles had prevented the Israeli forces from attacking PLO concentrations in central Lebanon. Now, the defense minister said, the Israeli Air Force can provide adequate cover for ground troops that are attempting to destroy the guerrilla military infrastructure in the central region.

The deployment of the missiles last year triggered a "mini" war of attrition across the Lebanese-Israeli frontier that led to an agreement by the PLO and Israel, arranged by U.S. envoy Habib, to cease hostile activities.

A spokesman for the Israeli Foreign Ministry said tonight that the attack on the missiles was necessary because Syria had taken advantage of Israel's declared intention not to engage Syrian ground forces in Lebanon.

"Assad knew that we were not going to attack the Syrian forces in Lebanon, because we had promised so. Because of this promise, he sent reinforcements to the area, and last night he sent new missile batteries to the Bekaa. This prevented us from pursuing the PLO in that area, so we had to destroy the missiles," the Foreign Ministry spokesman said.

Israeli military officials said that in addition to the 22 downed Migs, seven other Syrian aircraft were reported hit in the aerial battle. No Israeli jets were downed, the Army command said.

Israeli officials would not say how many Israeli aircraft were involved in the dogfights, but Damascus radio claimed that 96 Israeli jets attacked the missiles, and that 19 of them were shot down by Syrian aircraft and ground fire. The radio said 16 Syrian jets were downed.

Meanwhile, the Israeli military command announced tonight that the Mediterranean coastal town of Damour, 10 miles south of Beirut, had been captured by the invasion force and that its Palestinian occupants had fled.