Israeli armor, backed by artillery, today invaded West Beirut and nearly seven hours after their initial attack appeared poised to complete a pincer movement to separate the southern suburbs with their Palestinian refugee camps from West Beirut proper.

Attacking up the coastal highway Israeli units advanced to within just over one mile of the main avenue linking the Mediterranean with the National Museum, according to state-run Beirut radio.

By 7 a.m. Beirut time other Israeli units had advanced westward 600 yards in their farthest thrust from their starting point at the Museum crossing on the Green Line.

That line divides the capital into predominantly Christian East Beirut and mainly Moslem West Beirut.

From the museum the invaders advanced straight along a broad avenue, moving aside rudimentary Palestinian earthworks, and stopped at least temporarily near the Barbir Hospital.

At another attack point along the three-mile long Green Line, in the Beirut port area, the Israelis reached the Fattal office building just beyond the Western entrance to the port.

Several hours after the attacks began, artillery exchanges had intensified to a steady roar as the level of fighting increased. Observers here were particularly struck, however, by the absence of Israeli air power more than 3 1/2 hours after daybreak.

In Jerusalem, the Israeli military spokesman was quoted on the radio as saying that "this is not an attempt to capture the city," but a response to repeated violations of the cease-fire by the Palestine Liberation Organization, Washington Post correspondent Edward Walsh reported. As a result, the Israeli Army is "tightening the siege," he said.

Israeli Defense Minister Ariel Sharon hinted recently that Israel might move first on the Palestinian refugee camps concentrated in Beirut's southern suburbs before deciding whether to push into heavily urbanized West Beirut.

It was unclear what effect the Israeli move will have on negotiations to secure the exodus of the trapped PLO guerrillas from Beirut.

Special correspondent Nora Boustany, who witnessed from East Beirut part of the advance across the Green Line, said the Israeli armored column stretched for hundreds of yards.

One Israeli soldier said that the only early opposition at the Green Line was from snipers and light machine gunfire, Boustany reported.

The drive from the National Museum on the east side of the line, began taking form shortly after midnight. Monday, an entire Israeli armored brigade was positioned within striking distance of the Green Line. About 40 tanks and 150 armored personnel carriers were moved discreetly onto side streets and underground garages in and around the museum.

Shortly after midnight they were heard rumbling across and at 2:30 a.m Beirut time the end of the convoy had gone beyond the museum point, Boustany said.

Palestinian sources in West Beirut said the Israelis also had tried to breach the line near Galerie Semaan, the third major crossing point.

The Israelis first went in with minesweepers and earthmovers to remove the piles of sand on the side of the roads while Israeli tanks stood by.

Israeli soldiers took out loudspeakers and started calling out in Arabic telling the local population to leave and indicating which routes to take out of West Beirut. The soldiers shouted, "Run for your lives; clear the battle zone," Boustany reported.

Elsewhere, heavy artillery duels continued, with Israeli gun positions in the hills overlooking the southeastern suburbs pounding away at Bourj Brajneh and Chatila camp. Palestinians fired back with Katyusha rockets towards the Israeli position.

Gunboats continued shelling the coastline, lighting up the skyline as it appeared from a vantage point on the roof of the Alexandre Hotel.

Wafa, the Palestinian news agency, reported heavy Israeli naval gunfire directed at a Palestinian stronghold controlling the western entrance of the port, where the Israelis have increased their previously meager armor concentrations in the last two days.

Wafa also reported Israeli armored units attempting to advance northward from their positions near the airport under cover of heavy and intensifying artillery and rocket fire.

Despite the apparently light Palestinian resistance, military specialists have warned that the Israelis could yet face a difficult situation and take heavy casualties.

They reasoned that the Palestinian guerrillas, equipped with rocket-propelled granades and other infantry weapons, would be at a practical advantage in dealing at close quarters in built-up areas against Israeli armor.

Some specialists have estimated the Israelis could suffer 600 or more dead and wounded. So far the Israelis have lost more than 300 killed since the invasion began in southern Lebanon June 6.

Witnesses at a vantage point in East Beirut said fires were raging along the Mediterranean coast near the airport and a large blaze could be seen in West Beirut along the main avenue dividing the downtown districts from the southern suburbs.

They said machine-gun fire could be heard from the main crossing point between East and West Beirut by the National Museum, along with the sound of tanks moving on the streets.

Two persons who walked down close to the museum crossing said that the Israelis were using earth movers there. Israeli Knesset member Amnon Linn said in East Beirut that the Israelis had moved across the Green Line, but this could not immediately be confirmed.

Palestinian forces were firing at Israeli positions with Soviet-supplied Katyusha rockets.

Yesterday, Israeli gunboats, tanks and artillery throughout yesterday again shelled the southern suburbs and Western diplomats and Lebanese politicians voiced pessimism that a tenuous cease-fire would hold.

The officials had expressed fears of another major military pounding along the lines of Sunday's 14-hour attack, which caused widespread destruction. One suggested scenario was a "salami" operation to slice off more suburban territory as the Israelis did in and around the airport on Sunday and Monday.

The Israeli thrust came as growing--although unsubstantiated--reports said that negotiations through Lebanese proxies between the Palestinians and U.S. special envoy Philip C. Habib were advancing "rapidly and satisfactorily," according to one government source.

A Western envoy said the negotiators were down to dotting the i's and crossing the t's, but the main stumbling block remained conflicting Israeli and Palestinian demands that the other commence the complicated withdrawal exercise.

Lebanese leftists said Habib wanted the Palestinians to start the evacuation process by sending about a thousand guerrillas out of Lebanon.

The international force would bring into West Beirut an equal number of troops and then the process would be repeated until the withdrawal of all Palestinian forces was completed, they said.

Canadian Ambassador Theodore J. Arcand, who remained in West Beirut for weeks after all other Western ambassadors had gone, was ordered Monday by his government to leave.

In Jounieh, 11 miles from Beirut, Arcand told The Associated Press Tuesday that after the bombing Sunday, he and his staff surveyed 55 separate areas that had been hit. He said he considered none of them as attacks on Palestinian targets, as Israel has described its raids.

["If a Palestinian person--a man, woman or a child--were in the building, then I suppose you could call it a Palestinian target, but that's not how you or I would define it," he said.]

[He said the destruction caused by Israel's bombardment would "make Berlin of 1944 look like a tea party."]