Israeli forces have advanced to the Litani River, leaving this ever shrinking salient the only part of southern Lebanon still in Palestinian hands.

Despite a U.N. call for cease-fire, Israeli and Palestinian forces yesterday slugged it out around this Mediterranean port amid reports that the Palestinians had sworn to die to the last man to defend the 16 square mile salient.

Tyre was all but deserted of its 70,000 Palestinian and Lebanese inhabitants.

Israeli armor and infantry inched forward from hilltop and coastal plain positions, backed by heavy artillery and air and naval strikes.

Despite Israeli artillery and air strikes - which hit the last remaining bridge over the Litani in guerrilla hands with shrapnel - reporters crossed it at 5 p.m. on their way back to Beirut on the main coastal road.

Palestinian reports said the Israelis, backed by tanks and other armor, were heading toward Hammideyeh, which is less than two miles from that road.

The Israeli decision not to take Tyre and the much battered Rashidiyeh Palestinian refugee camp to its immediate south came as no surprise. Analysts had doubted all along the Israelis were willing to pay the price in casualties, especially since they controlled the whole salient from the commanding heights of a range of hills less than three miles from the coast.

Informed sources did not expect an effective U.N. force to be on the ground in southern Lebanon for another 10 days or two weeks - or the Israelis to begin withdrawing before its arrival.

Any such delay needs not necessarily favor the Israelis, who have been considerably hindered by Palestinian hit-and-run attacks behind lines thought to be under control.

Indicative of Israeli problems were the estimated 55 men the sources said the invasion force has lost so far - three times the 18 officially admitted by the Israelis.

At the very least, the sources credited the Palestinians left behind south of the Litani with enough staying power to cause trouble for the casualty-sensitive Israelis for at least 10 days.

Symptomatic of Palestinian tactics was a claim that they killed 30 Israelis when they destroyed four trucks in a village near Tyre.

Another sources of potential military and political danger was represented by the arrival of a convoy of armsbearing trucks - 15 Syrian and 31 Iraqi.

They were the first concrete proof of some measure of Arab solidarity for the Palestinians, who have accused their erstwhile allies of abandoning them when they were needed most. The Iraqi shipments were also a kind of Inter-Arab oneupmanship which could lead to a possible reconciliation between arch rivals Syria and Irq, who hate each other as much as they late Israel.

Iraq refused last December to join the radical front led by Syria and designed to counter the Egypt's now faltering peace negotiations with Israel. Without Iraq's oil revenues and 188,00-man army, that front has proven ineffective.

Iraq's arms shipments were proof of its hardline credentials - and forced Syria not only to allow the Iraqi trucks transit rights but to send arms of its own to the Palestinians.

Despite such pro-Palestinian gestures, however, diplomats here suggested that any overall settlement in the south eventually could well involve Syrian peacekeeping forces in Lebanon moving in to further reduce the dwindling autonomy of the guerrillas.

One scenario circulating here first calls for moving the U.N. force into the region south of the Litani to stop the shooting and allow Israeli troops to pull back closer to the border.

Then the Syrians would be encouraged to move down from the present "red line" - beyond which the Israelis objected ever since the Arab peacekeeping force was formed in 1976. They would control the remaining 200-square-mile Palestinian redoubt. It runs from Zahrani to the Litani on the Mediterranean to Nabatiyeh and the Litani in the center of Lebanon.

But no matter how tempted the Syrians might be to bring the Palestinians to heel, analysts here doubted that Damascus could afford to be seen collaborating at this point in a scheme viewed here as being so openly favorable to Israel.

Whatever Arab governments may think of the Palestinian raid in Israel that set off the Israeli invasion - or of the Palestinian guerrilla movement in general - the commandos spirited defense, now in its sixth day, has once again made them heros in Arab eyes.

For the moment, that has blurred the fact that the Israelis have been able to kick the Palestinians out of their last remaining turf and capture underground arms and ammunition dumps running into thousands of tons of machine guns, anti-personnel and anti-tank mines and other light weapons in quantities sufficient to supply an entire Arab army.

Also forgotten for the time being is the Palestinian military's underestimation of the size and scope of the Israeli attack, sources said. Apparently, only the usual reprisal raid had been expected.

The eventual price the Palestinians may end up paying for this failing is being forced to settle for tiny enclaves, one south of the Beirut-Damascus road along the slopes of Mount Hermon, the other a training area around Nabi Spat in the Bekaa Valley further north.

But it is also hard to see what the Israelis have really gained by the invasion. The Palestinian main force is reported intact and its heavy weapons positioned at a new defense line running from Nabatiyeh to Sarafand on the Mediterranean.

Palestinian control of the coastal highway in that area could cause serious problems for the U.N. force which must use it.

Moreover, informed sources speculated on the U.N.'s abilitty to handle any showdown with the Palestinians, whose armaments outclass the buffer force's arms, the heaviest being machine guns, mortars and recoilless rifles.

Nor is there any immediate hope of sending the fledgling Lebanese army, only now recovering from the ravages of anarchy during the 1975-1976 civil war, into the south any earlier than four to six months from now.

In the meantime, Israel is expected to beef up its Christian allies in the south and launch an intensive program to win over returning villages.