Israeli warplanes bombed and rocketed targets along a 21-mile stretch of Lebanon's coast today, causing heavy casualties and damage.

The raid coincided with a report by Israel Radio that the Israeli Cabinet, conferring with hospitalized Prime Minister Menachem Begin by telephone, rejected a U.S.-Soviet compromise to move an existing U.N. force that is now manning positions along Israel's northern borders into the Sinai Peninsula between the Egyptian and Israeli armies.

Semiofficial Egyptian newspapers, meanwhile, reported that Cairo has accepted the compromise proposal.

The mandate of the U.N. Emergency Force as a buffer between Israeli and Egyptian units in the Sinai expires Tuesday, and the Soviet Union has said it would veto attempts in the United Nations to extend the mandate.

Israel had been expected to agree to a U.S. proposal, which reportedly has Soviet acquiescence, to move members of the U.N. Truce Supervision Organization into the Sinai.

According to the radio, however, the Israeli Cabinet rejected the plan because the U.N. Truce Supervisory Organization has Soviet members, and Israel wants only units from countries with which it has diplomatic relations to be based in the Sinai during its withdrawal there.

Official sources in Jerusalem said Foreign Minister Moshe Dayan had sent a cable to Secretary of State Cyrus Vance on the question of using unarmed U.N. observers to supervise the peace treaty in the Sinai, but they refused to disclose the cable's contents or confirm the radio report.

In Beirut, radio reports said Israeli warplanes struck five towns and a highway filled with weekend travelers. An Israeli military spokesman in Tel Aviv said the targets were Palestinian "terrorists concentrations."

The state-run Beirut Radio reported "heavy casualties" - at least 15 people killed and 50 wounded in the raid. Unofficial reports and hospital sources said the tolls were higher.

According to the rightist Phalangist radio, Israeli jets pounded the towns of Damour and Naameh, 13 miles south of Beirut, then swept farther south to bombard Sarafand, Khaizaran and Adloun another eight miles down the coast.

The planes also attacked Lebanon's coastal highway linking the capital with the south, Beirut Radio said. It said the road was filled with travelers returning from outings on a hot summer Sunday.

Numerous cars were blown up and heavy damage was reported at shops along the highway between the Zahrani River and the costal city of Tyre.

In Tel Aviv, a military spokesman said the Israeli planes all returned safely after the raid. The announcement, which did not specify the locations of the guerrilla bases, said it was the first Israeli air raid on Palestinian targets since a Syrian-Israeli dogfight over Damour June 27.

Israel said then that its planes shot down five of Syria's Soviet-made Mig 21 jet fighters. Syria conceded losing only four and said its fighters shot down two Israeli planes. Israel denied losing any aircraft.

Two of today's targets, Damour and Naameh, formerly were inhabited by Lebanese Christians, but were taken over by Palestinian guerrillas and refugees during the 1975-76 Lebanese civil war.

The Palestinian news agency WAFA said the Israeli warplanes swung south across Beirut, then dived on their targets from darkening skies at dusk.

Palestinian sources in Sidon said a number of people were believed to be buried under rubble at Sarafand, a few miles from Sidon.

Hospital sources in southern Lebanon said most of today's casualties were civilians.

There were no signs of any attempt by Syrian fighters to intercept the Israeli planes as they tried to do in the June 27 clash. Syria maintains about 25,000 troops in Lebanon as an Arab League peacekeeping force.

Israeli Air Force chief David Ivri said last week that air raids in Lebanon would continue and that the Syrians had been careful since last month's dogfight to keep clear of the frequent Israeli reconnaissance flights over southern Lebanon.

WAFA said the planes used in today's raid were American-made F4 Phantom jets. The bried Israeli military communique issued in Tel Aviv did not say what type of planes were involved.

In the June air raid, the Israelis used their advanced U.S. made F15s, drawing an expression of "serious concern" from Washington, which said the planes had not been provided for aggressive purposes. CAPTION: Map, no caption, By Dave Cook - The Washington Post