Syria moved Soviet-built surface-to-air missile batteries into eastern Lebanon today, according to witnesses, in an apparent response to Israel's downing two Syrian Army helicopters in the nearby Bekaa Valley yesterday.

Reporters on the scene said two Israeli jets flew over -- and presumably photographed -- the SA2 and SA6 missile sites only hours after they were driven in from Syria at first light.

The missiles' introduction represented the most serious escalation in the current crisis between Syria and Israel, according to diplomats who recalled that previous tests of will between them had triggered Arab-Israeli wars in 1967 and 1973.

The presence of the batteries appears to violate the tacit understanding under which Israel did not respond militarily when Syrian troops entered Lebanon in 1976 to help end the civil war. One of the conditions reportedly imposed by Israel was that no ground-to-air missiles were to be stationed in this country.

Diplomats suggested Syria apparently moved in the batteries after judging that Israel had already violated the accord by shooting down the helicopters and what Syria said was the strafing and bombing of its troops in mountain positions yesterday. Israel denied hitting ground targets in the mountains.

In the south, Israeli war planes once again carried out massive air attacks against suspected Palestinian guerrilla positions in and around the market town of Nabatiyeh and the old Crusader fortress at Beaufort.

Judging from the results of other raids Sunday and Monday in the south, the Israelis may be using the attacks into Lebanon to destroy many of the perhaps 60 World War II-vintage Soviet-built T34 tanks in the Palestine Liberation Organization arsenal. An Israeli raid near the Mediterranean port of Sidon was said by informed sources to have destroyed at least 10 T34s.

After yesterday's air strikes, Deputy Israeli Defense Minister Mordechai Zippori said his government "was making it clear that in Lebanese skies there will be no Syrian air activity."

An at least one SA2 and a dozen SA6 missile batteries were clearly visible to reporters who traveled along a road linking the Lebanese air base at Rayak to the main Beirut-Damascus highway only four miles from the Syrian border.

The two downed helicopters were based at Rayak and other Syrian helicopters were reported to have used the base again today apparently to resupply Syrian troops on the mountaintops on the other side of the Bekaa Valley.

The SA6 missiles, with radar trucks and in some cases still mounted on tracked vehicles, were being moved into emplacements behind fresh earthworks less than 15 yeards from the road, the reporters said.

In theory, the surface-to-air missiles in eastern Lebanon now could provide protection against Israeli planes all the way south down the narrow, fertile Bekaa Valley to the Israeli border town of Metulla, military specialists said.

Diplomats, rasing the qluestion of Israeli reaction to the Syrian move, also noted the possibility that Soviet advisers accompanied the missile batteries into Lebanon since the Syrians are not thought to be trained in handling the complicated radar and other electronics gear normally accompanying the weapons.

If indeed soviet advisers were present in Lebanon, these diplomats asked whether that meant the terms of the Soviet-Syrian treaty of friendship and cooperation of last fall covered the 22,000-man Syrian force that has been charged with keeping the peace here since 1976.

Military specialists suggested Syria had moved the missiles into the Bekaa Valley because topographical features prevented total protection of its forces there despite the sophisticated air defense systems Syria is creditred with possessing just inside its own borders.

Meanwhile, Syrian Foreign Minister Abdul Halim Khaddam declared himself "satisfied" with his two days of talks here with Lebanese politicians aimed at ending the now 4-week-old round of fighting.

Khaddam's mission, which originally was conceived to press home Syria's military advantage against the Christian militias, suddenly lost much of its impetus as a result of the Israeli air strikes.

Christian politicians, including Information Minister Michael Edde, today attacked Israel for complicating Syrian-Lebanese rapprochement. But privately the same politicians conceded that their public statements were designed to minimize the stigma attached to the Israeli connection in much of the Arab world.

Some Lebanese commentators feared that Israel would not permit the kind of settlement between the Christian militia and Syria likely to provide even an interim period of stability. Recalling successful Israeli efforts to break just such an alliance in 1977 and 1978, a veteran editorialist said, "We just cannot believe the Israelis are really interested in a peaceful Lebanon. In our eyes, they do not fill the bill as the salvation army."