Israeli warplanes pounded Syrian and Palestinian positions in and around Lebanon's Bekaa Valley for more than seven hours today in Israel's biggest military operation since the end of the siege of Beirut a month ago.

The Israeli planes, flying in pairs that were often in sight of the Lebanese capital, struck repeatedly at targets in a wide belt of the Syrian-held Bekaa and along the Beirut-to-Damascus highway that leads into the strategic valley through a range of mountains east of Beirut.

The massive air strikes today were the fourth time in six days that Israel has sent its Air Force into action against Syrian and Palestinian targets east of the Lebanese capital.

An Israeli military spokesman, announcing today's air raids several hours after they began at 7:30 a.m. local time, said they were in reply to 98 violations of the July 22 cease-fire between Syria and Israel. He said that 12 Israeli soldiers had been killed, 20 wounded and nine taken prisoner in the course of the violations.

The spokesman said that a Syrian SA9 antiaircraft battery was destroyed in today's attacks, which hit Syrian and Palestinian armored personnel carriers, heavy artillery emplacements, machine gun positions and other military targets.

A total of six of the, four-rocket SA9 batteries were destroyed in the three previous air raids.

The Israeli military spokesman said today that several PLO offices in the Bekaa also had been hit, including the headquarters in the valley town of Shtawrah of the Popular Front for the Liberation of Palestine -- General Command, the PLO splinter group that last week claimed credit for capturing eight Israeli soldiers from an observation post above Bhamdun.

Today's strikes hit a total of 15 different locations around the Bekaa Valley, according to the national Beirut radio. The raids were concentrated around Shtawrah, surrounding villages and the heights above it as well as the mountain road that descends into the valley from the Israeli positions at Bhamdun.

Beirut radio estimated that between 40 to 50 persons were killed, but there was no independent verification of this figure because the road through the Bekaa was cut, as were all direct communications with the towns that were hit.

Damascus Radio reported tonight that only two of its soldiers were wounded, and a few artillery pieces damaged. It said of the raids that "Israel repeatedly violated the cease-fire and made a total of 227 sorties during which Syrian positions in Lebanon were bombed."

Tension has been mounting in the area of today's attacks since the Palestine Liberation Organization withdrew 11,000 guerrillas from Beirut last month under a U.S.-engineered agreement.

That accord, negotiated over the summer-long Israeli siege of Moslem West Beirut by U.S. special envoy Philip C. Habib, was supposed to be only the first phase of a larger accord to be negotiated this fall to try to get all Syrian and PLO forces to withdraw from the rest of Lebanon--the Bekaa Valley as well as the north of the country -- in exchange for Israeli withdrawal from the southern half of the country it has occupied since its June 6 invasion.

With fears rising that Israel's bombing attacks today could presage a move to try to push the Syrians out of the Bekaa by military force rather than political negotiations, Western diplomats here saw the move more as one of bargaining pressure as U.S. Deputy Assistant Secretary of State Morris Draper, who is to conduct the next phase of the peace negotiations, prepares to return to Lebanon this week.

Israel and Syria have said they would not agree to withdraw their forces from Lebanon unless the other agrees to do so first. The PLO, which has strengthened its own forces in northern Lebanon and the Bekaa since completing its withdrawal from Beirut Sept. 1, has given no indication that it would leave Lebanon too, though its position depends on what Syrian President Hafez Assad decides to do about his own military.

Protracted negotiations are predicted once Draper renews talks with Syria, Israel, and, through intermediaries, the PLO. Syria and Israel have massed troops and large forces of tanks along the cease-fire lines that were proclaimed July 22 after the Israeli Air Force decimated Syria's Air Force by knocking out close to 100 of its Russian-made jet fighters and bombing into oblivion their SAM antiaircraft systems installed in the Bekaa.

Renewed Israeli air attacks on Syrian positions began last week in and around Dahr el-Baydar along the Beirut-to-Damascus road, after Syria had moved up new SA9 batteries behind their lines facing the Israeli forward positions at the town of Bhamdun, 15 miles east of Beirut. Israel has repeatedly warned Syria that it will not allow it to reinstall SAMs in Lebanon.

In Washington, U.S. officials declined to say whether they consider the movement of Syrian missiles inside Lebanon a violation of the agreement worked out by Habib. Instead, White House spokesmen Larry Speakes and State Department spokesman Alan Romberg said the U.S. focus is on an early withdrawal of all foreign forces from Lebanon and all factions there refraining from provocative moves.

Though Syrian positions were hit today as they have been in recent days, the Israeli attacks seemed more intent on blunting operations against them from the PLO in the Bekaa. Israel claims the PLO has massed from 4,000 to 5,000 men behind the Syrian lines since their withdrawal from Beirut. Other Western diplomatic estimates here put that figure up to twice that high.

Israel has held Syria responsible for the PLO actions from behind their lines such as the capture of the eight soldiers a week ago and the ambush of an Israeli jeep last Friday that left three Israeli soldiers dead and one wounded.

A spokesman for the Israeli military command said today that the PLO had been operating "with Syrian knowledge and protection and full consent" and that was something that "Israel cannot agree to."

The Israeli military chief of staff, Gen. Rafael Eitan, claimed last week that Syria had massed three mechanized divisions with up to 1,300 tanks in the valley while holding another division with 300 tanks in reserve over the border. The Israeli military today said the PLO was turning the Bekaa into a new base for itself behind the Syrian force to replace those strongholds it had lost in southern Lebanon and Beirut because of the Israeli invasion.

Though U.S. diplomats here are still privately confident that Israel does not want to resume open warfare with the Syrians in the Bekaa, Israel's warning yesterday that it will tolerate neither repeated cease-fire violations nor stand for a war of attrition in Lebanon was being viewed with serious concern.