Syrian forces shot down an Israeli F4 Phantom fighter-bomber over eastern Lebanon today following Israeli air strikes against Palestinian targets in West Beirut and new Syrian missile sites in the Bekaa Valley, according to military communiques from the opposing sides.

The plane, which Israel indicated was on a reconnaissance flight, was the second that Israel has admitted losing since the June 6 invasion of Lebanon. An American-supplied Skyhawk was shot down over southern Lebanon by Palestinian guerrillas in the early days of the war and its pilot captured. Israel claimed to have downed more than 85 Syrian warplanes, more than a quarter of the Syrian Air Force's operational strength, in dogfights last month.

As the Israeli planes bombed Palestinian strongholds on the outskirts of West Beirut and its southern suburbs for the third straight day, Lebanese President Elias Sarkis for the first time publicly condemned the Israeli occupation of the southern third of Lebanon, denouncing "abuses" by the invasion forces.

In a separate statement, Prime Minister Shafiq Wazzan declared that the Lebanese government had rejected a proposal for a political settlement of the crisis, in which 5,000 to 6,000 Palestinian guerrillas trapped in Beirut would be sent to eastern and northern Lebanon until a country of refuge could be found.

"We have asked that the Palestinian fighters evacuated from Beirut be sent out of Lebanon," Wazzan said following a special Cabinet meeting. He said the rejection of a temporary transfer to other parts of the country had been communicated to U.S. special envoy Philip C. Habib, who conferred with Saudi leaders today before moving on to Cairo in a tour to promote a political solution.

The downing of the American-made Israeli Phantom over the eastern Bekaa initially was reported by the Palestinian news agency Wafa and the radio of the rightist Christian Phalangist militia. It was confirmed later by an Israeli military communique, which said the plane was shot down after raids in which Israel claimed to have knocked out three Syrian batteries of new Soviet-made SA8s, each with four of the surface-to-air missiles.

Later, Washington Post correspondent David B. Ottaway reported from Jerusalem that Israeli television said the plane was hit from the Syrian side of the border by an older SA6. The military did not confirm this, however, and it was speculated that a new SA8 on the Lebanese side and missed by the Israelis may have downed the plane.

The Israeli military spokesman said the SA8s were "the most modern and most mobile" Soviet missiles, using two kinds of radar and operating directly from vehicles. He said the only comparable missile being used by NATO forces was the German Roland.

In the first week of the war, Israeli aircraft apparently using advanced radar-homing missiles, knocked out as many as 19 batteries of Soviet-supplied SA6, SA2 and SA3 missiles in the Bekaa and inside Syrian territory.

Israel said the two fliers in the Phantom bailed out and were captured by the Syrians, although the official Syrian news agency said one was captured and the other was found dead.

Before the announcement that the Phantom had been downed, a Syrian communique warned that if Israel continued to violate the June 11 cease-fire between the two sides, "Syria will respond with sudden and surprising blows and with all types of weapons" to inflict heavy losses in lives and equipment.

The Bekaa strikes coincided with raids over the capital in which Israeli planes pounded the Cite Sportif stadium and Palestinian neighborhoods and camps in the southern suburbs of West Beirut. Israel said the strikes were directed against Palestinian ammunition dumps and tank and artillery positions.

The air raids followed artillery exchanges this morning for two hours in the southern suburbs and Palestinian claims to have mounted two commando attacks behind Israeli lines last night.

The Palestine Liberation Organization asserted that two Israelis were killed and four wounded last night near the Israeli-occupied port city of Sidon in southern Lebanon when their halftrack went over a landmine. It also said guerrillas killed or wounded a dozen Israeli soldiers in an ambush near the mountain village of Alayh, 12 miles southeast of Beirut.

Following the air raids, Israeli gunboats reportedly fired on the Palestinian refugee camp of Burj al Barajinah near the Beirut airport and at Palestinian and Lebanese leftist positions at Ouzai on the coastal road south of the capital.

The PLO's news agency said 51 persons, mostly civilians, were killed or wounded today by Israel's land, sea and air bombardment. Reliable independent casualty estimates could not be obtained. Wafa also said 17 Israelis had been killed or wounded today.

While these figures could not be confirmed either, a Lebanese Cabinet minister said Israeli casualties recently have been higher than admitted in Tel Aviv. Quoting medical sources in Sidon, he said that 11 Israelis were killed in a guerrilla attack during the night of Wednesday to Thursday on an Israeli-occupied government office complex. Israel has not mentioned any casualties in the incident, which led the occupation forces to seal off the city, impose a curfew and round up dozens of residents for interrogation, according to reports from Sidon.

In a statement to the Cabinet today, President Sarkis strongly condemned what he called Israeli encroachment on Lebanese internal affairs, such as the civil administration, the Lebanese Army and security forces. The comments were later broadcast by the state television.

"Israeli occupation forces in southern Lebanon paralyze the official administrations and form parallel administrations," Sarkis said. "They also attack Lebanese Army barracks, soldiers and members of the Internal Security Forces and arrest some of them."

The president, a Maronite Christian who lives in an Israeli-occupied suburb of Beirut, said such "abuses" would be reported to international bodies. Lebanese sources later said the government would officially inform the U.N. Security Council.