At least three Israeli soldiers were killed by Palestinian guerrillas and two others were reported missing yesterday when their jeep crossed into Palestinian-held territory south of the biblical port city of Tyre in southern Lebanon, Israeli authorities said.

They were the first Israeli casualties reported since a cease-fire was declared March 21 following the Israeli invasion of southern Lebanon. The incident marked the most serious breach of an uneasy calm that has prevailed since U.N. peacekeeping forces moved into the area.

There were conflicting reports of the exact number of Israelis killed, with U.N. officials in New York saying they had information from Beirut that the Palestine Liberation Orgainization had the bodies of five Israelis.

The Israeli Embassy here said it had received word from the Israeli Army in Tel Aviv that three were known dead, two were missing and believed wounded and two others were wounded but managed to make it back to Israeli lines.

Concidentally, U.N. Secretary-General Kurt Waldheim, visiting London, sent a personal message to Israeli Prime Minister Menachem Begin urging a "speedy implementation" of the withdrawal of Israeli troops from south Lebanon. The letter was unrelated to the Palestinian attack, U.N. officials said.

Israeli U.N. Ambassador Chaim Herzog said the Tyre attack "highlights the problem we face . . .we don't want to leave a vacuum there.

"Those who are pressing for an Israeli withdrawal before the U.N. is ready to take control are taking on a tremendous moral reponsibility because, in fact, they are sentencing (Lebanese civilan) Christians to a massacre," Herzog said in a telephone interview.

The Israeli military command said that six Israeli soldiers and one civilian were riding in the vehicle about 5 p.m. Wednesday when it strayed over the Palestinian line at Ras Al Ain, about three miles south of Palestinian-held Tyre.

The bodies of three soldiers were left in the vehicle, while the other four men - all wounded - ran toward the Israeli side. Two made it safely back. It was not immediately known if the civilian was among the missing.

PLO officials in Beirut said Israeli gunners retaliated with heavy shelling of Palestinian positions around the port city.

Spokesman for the PLO in Tyre and Beirut offered conflicting versions of the incident, with some claiming they had the bodies of four Israelis and that they had captured a fifth soldier.

A PLO military communique from Beirut said the Israeli vehicle had attempted to advance toward a guerrilla position.

A senior PLO official in Beirut was quoted by United Press International as saying the terrorist organization wanted to exchange the bodies for Palestinian prisoners. He cited as a precedent a similar exchange following a 1968 clash at the Jordanian town of Karameh.

Israeli Embassy officials here said the Israeli Army in Lebanon had begun a search for the missing soldiers, and had asked the U.N. forces and the International Red Cross to do the same.

A U.N. spokesman in New York said Waldheim's message to Begin, sent through the Israeli Embassy in London, observed that deployment of the U.N. Interim Force in Lebanon (UNIFIL) was continuing and that the secretary-general was "especially concerned" about the withdrawal of Israeli troops.

"Waldheim "hopes there will be early and positive action by Israel and that the conditions will soon emerge in which the U.N. Interim Force in Lebanon will be able to fully carry out its mandate," the spokesman said.

However, Israeli Ambassador Herzog said the did not intrepret the message as reflecting impatience or urgency, saying Waldheim "just wanted to share his concern . . . he expressed an appreciation of the Israeli desire not to leave a vacuum."

Another Israeli official at the U.N. mission expressed a sense of frustration at what he termed the public perception that Israel was dragging its feet in withdrawing from south Lebanon.

"So far, UNIFIL has been a fiction - Well, not a complete fiction, but it is just getting itself together. Withdrawal is not unilateral, it is not entirely dependent upon us," said the official, who asked that he not be identified.

"We clearly see Resolution 425 as mandating our withdrawal, linked with the establishment of UNIFIL. The two must go hand in glove," he added.

The U.N. Security Council Resolution, adopted March 19, calls for the withdrawal of Israeli troops and the restoration of the authority of the Lebanese government in the area.

Israeli mission officials said the government has consistantly kept the United Nation advised about its withdrawal intentions, and a U.N. spokesman in Tel Aviv confirmed yesterday that Israel has given the United Nation a specific plan for withdrawal from the area secured by the peacekeeping force.

The plan, presented by Israeli Chief of Staff Lt. Gen. Mordechai Gur in Jerusalem, would affect Israeli troops in the eastern half of the 30-mile front and take place in two stages on April 11 and April 14, the Associated Press reported.

The Israelis would not leave Lebanon, but would pull back behind positions occupied by the U.N. force, the U.N. announcement said.

The projected strength of the U.N. peacekeeping force is 4,000, about half of which is in position in the sourth.

The acknowledged Israeli Army death total since the invasion, including the latest victims, now stands at 24, including 21 soldiers killed in action.

The incident near Tyre was the third in an many days involving Israelis and Palestinian guerrillas. An Israei soldier was shot to death near the walled Old City of Jerusalem, and 24 hours later an Arab youth was slain when an Israeli Civil Defense fired on him in the same area.

Meanwhile, a survey conducted by the Red Cross claimed that 80 per cent of the towns and villages in southern Lebanon were damaged by the Israeli invasion last month, according to a Reuter report from Beirut.

Although the figures have to been released, a source was quoted as saying 82 villages were damaged, and six of them almost destroyed. More than a thousand people, most of them Lebanese civilians, were killled, according to the estimates.