Israeli warplanes and artillery pounded Palestinian and Syrian targets from West Beirut to the Syrian border today in apparent retaliation for mounting Israeli casualties in Lebanon. Palestinian and Syrian forces fought back and later cited heavy civilian casualties in the city and destruction of Israeli armor in the Bekaa Valley.

The attacks were by far the most serious breach of a series of cease-fires, the first of which came into effect on June 11.

Israel's air strikes against Palestinian neighborhoods and positions in West Beirut and its southern suburbs were the first since June 25 and signaled a sharp escalation of Israeli pressure for the evacuation of 5,000 to 6,000 Palestinian guerrillas trapped in the Moslem half of the Lebanese capital.

Israeli spokesmen indicated the attacks were intended as retaliation for "75 violations" of the cease-fire in the last two weeks, culminating in the death Wednesday of five Israeli soldiers in an ambush by Palestinian guerrillas near Qirawn Lake in the Bekaa Valley. Israeli losses in the new fighting were listed as two killed and two wounded. Sources stressed the "limited" nature of the 90-minute actions and the military said after the attacks ended that Israel would again observe a cease-fire. Story on Page A23.

The raids came shortly after U.S. special envoy Philip C. Habib left by helicopter for Cyprus to begin a tour of Syria, Saudi Arabia, Egypt and Israel in pursuit of a settlement in Lebanon. From Cyprus, Habib flew to Damascus for what Syrian officials called "urgent and very important talks" with President Hafez Assad.

Habib's departure after six straight weeks here left negotiations on a political settlement of the crisis "at a standstill" and fueled concern about further military actions, according to a key mediator in the talks.

Former prime minister Saeb Salam said the trip was worrisome because there had been no mention that Habib would return to Beirut following his tour. On the other hand, Salam said, "it could end up facilitating things" if Habib could capitalize on any progress made in talks this week in Washington between President Reagan and the Saudi and Syrian foreign ministers.

The air strikes and shelling began in the Bekaa Valley in eastern Lebanon, where most of the more than 30,000 Syrian troops in the country are deployed along with thousands of Palestinian guerrillas and foreign volunteers. The Israelis hit targets along the Beirut-to-Damascus road from the village of Sofar, the westernmost Syrian position, to Masnaa, near the Syrian border, according to officials in Beirut and Damascus.

Among the sites reported bombed were the Bekaa villages of Bar Elias and Ksara and a Lebanese Army barracks at Baalbek.

Lebanese police listed an initial West Beirut toll of five killed and 12 wounded, while the Palestinian Wafa news agency put overall civilian casualties at 182.

Reuter quoted a military spokesman in Damascus as saying Syrian troops in the Bekaa Valley destroyed 24 Israeli tanks and 8 armored vehicles, while losing 16 tanks. The official SANA agency said two Syrian soldiers were killed and 14 wounded.

It was not immediately clear whether Israeli forces were advancing on the ground in conjunction with the strikes.

In addition to the Qirawn Lake ambush Wednesday, where six Israeli soldiers were wounded, five others were wounded near the southern city of Tyre when guerrillas fired on their jeep.

Earlier, guerrillas fired a Soviet-made Katyusha rocket into northern Israel from Israeli-occupied southern Lebanon.

Israeli spokesmen have said the guerrillas infiltrated from behind the Syrian lines, but the Palestine Liberation Organization in West Beirut claims it has fighters operating in Israeli-occupied areas.

The Israeli bombardment was by far the most serious breach of a nearly six-week-old cease-fire with the Syrians. Previous flare-ups included a limited Israeli air strike to take out new surface-to-air missile batteries that Jerusalem claimed the Syrians had moved into the Bekaa. Israeli warplanes, apparently using advanced radar-homing missiles, had destroyed as many as 19 Syrian missile batteries in the Bekaa and inside Syrian territory shortly after the Israeli invasion of Lebanon began June 6.

Shortly after today's air strikes in the Bekaa, Israeli warplanes flew over the Lebanese capital, staging mock air raids against targets on the southern outskirts of Beirut and drawing ineffectual antiaircraft fire.

About half an hour later, at 5 p.m., Israeli planes, believed to be American-supplied F4 Phantoms and F16s, started bombing targets in West Beirut's largely Palestinian Fakhani neighborhood, where some PLO offices are located. They also reportedly struck the Palestinian camps of Burj Barajneh and Sabra in the southern suburbs, a sports stadium and the formerly posh seafront district of Ramlet el Baidah.

Reporters for The Associated Press reported having seen three surface-to-air missiles fired from West Beirut, but they went well wide of their targets. Swooping low over the city, Israeli planes dropped thermal flares to divert the heat-seeking missiles.

The air raids, accompanied by Israeli tank and artillery fire on the southern suburbs near the Beirut airport, cracked the sixth cease-fire between Israeli and Palestinian forces around the capital. Despite repeated violations, it had been generally in effect since July 11, when Israeli forces unleashed the heaviest tank and artillery bombardment on West Beirut since the war began.

There were no reliable estimates of casualties today, and it was difficult to distinguish between the new damage and the old in the targeted neighborhoods, which have been pounded repeatedly from air, land and sea since the siege of Beirut started in mid-June.

As many as 10 Israeli planes were sighted in the air at one time today, but not many bombs were seen being dropped, and the overflight stopped after about 90 minutes.

While today's attacks seemed intended as a warning to speed up the slow pace of the negotiations to get the PLO out of Lebanon, there were indications that the Israelis might be preparing for military action to force such an evacuation.

According to a knowledgeable military source, the Israelis have brought most of their artillery into Lebanon and concentrated almost all of it around the capital. Of the approximately 100 artillery pieces targeted on West Beirut, about two-thirds are 155-mm howitzers and most of the rest are 175-mm guns, the source said.

He estimated that the guns were enough to demolish the Palestinian camps south of the city in one night, without resorting to air strikes. He said that yesterday, gun crews south of the airport appeared to be zeroing in on targets and preparing ammunition for an attack.