After slicing through and occupying nearly 30 percent of Lebanon within five days, Israel's armed forces today appeared to have made little progress on the ground, but they unleashed their most intensive air bombardment to date against Beirut.

Opposed by increasingly less dense antiaircraft fire from the capital, Israeli planes singled out Syrian targets such as the seaside secret police headquarters and ammunition depots.

The wail of ambulances rushing through the capital's increasingly deserted streets followed the air raids and naval gunfire from Israeli ships just offshore that began at dawn and continued until dusk.

The Israelis were thwarted in repeated overnight and early morning efforts to land troops near Beirut International Airport, which they seek to capture to press their campaign against the nearby shanty-town housing Palestinian refugees and offices of the Palestine Liberation Organization guerrillas' high command.

In one sequence of bombing runs, observed from the hills above the airport, the warplanes hit a Datsun car depot, the Pepsi-Cola bottling plant, a farm equipment warehouse and a tin can factory just to the east of the runways where four jetliners were parked.

Later, the Israeli aircraft bombed refugee camps at Bourj el Brajneh and Chatila Sabra, the guerrilla office near the Arab University and a previously spared neighborhood less than 200 yards from the dividing line between the Moslem and Christian sectors of the city.

Other targets included Syrian and Palestinian arms and ammunition depots, the water purification plant and the Shia Moslem center of Ouzai, along the coastline opposite the airport runways.

In clear blue skies, Israeli aircraft also bombed and rocketed along the vital Beirut-Damascus highway and in the Bekaa Valley near Syria where Israelis yesterday claimed to have destroyed Syria's surface-to-air missile sites.

Syrian President Hafez Assad sent a message to his troops in Lebanon praising their "steadfast" performance and reminding them of "our motto--Martyrdom or Victory."

In similarly defiant fashion, Bassam Abu Sharif, the spokesman for the Palestinians and their leftist Lebanese allies, warned that "Beirut will be the graveyard of Begin" because "we are prepared to fight the Israelis street by street, house by house."

At a news conference, Abu Sharif claimed the Israelis were lying about their casualties and losses and said the Palestinians and their allies had destroyed at least 150 tanks and other armored vehicles.

The unrelenting tempo of Israeli air strikes against Syrian targets contrasted oddly with a yellow leaflet dropped by Israeli aircraft over Beirut late this morning offering the Syrians safe conduct out of the capital if they left "within a few hours."

"We shall capture the city in a short period," the leaflet boasted. "We have committed a large part of our air, naval and ground forces" to the Beirut area "including a huge number of tanks."

Warning that opposing the Israelis was "tantamount to suicide," the leaflet, signed by Israeli Brig. Gen. Amir Druri, said, "We have no intention of fighting the Syrian Army." On the back of the leaflet was a map showing the two "safe conduct" exits for the Syrians.

But even before the leaflets were dropped, Israeli planes were attacking the Beirut-to-Damascus highway, one of their recommended escape routes.

In Paris, Syrian Ambassador Yussef Shakour dismissed as "psychological warfare" the Israeli demand that the Syrians evacuate Beirut, Washington Post correspondent Jim Hoagland reported. The ambassador said Syrian troops "will continue to do their duty. . . . Everyone should realize that this war has just begun."

A drive along the largely deserted highway in the early afternoon today took one past Syrian Army vehicles, including tanks, armored cars and three still fiercely burning oil tankers, all hit by Israeli rockets at wide intervals.

A number of knocked-out civilian vehicles also littered the road.

Government-run Beirut radio said the raids were the heaviest so far against Beirut.

For the first time since Israel started bombing Friday in preparation for the invasion, the Palestinian Red Crescent Society estimated the number of dead and wounded, putting the total at 10,000 but giving no breakdown.

The PLO intelligence department set the casualties so far at 8,000, most of them civilians.

Indicative of Syrian determination to fight back was the risky decision to move tanks up from the Bekaa Valley in broad daylight without air cover to the mountains where for the last two days Israelis and Syrians have slugged it out for control of the Beirut-to-Damascus highway.

In other ground fighting the Syrians were still holding the line in the southern Bekaa, which they consider their vulnerable flank for any Israeli attack on Syria.

The terrain is boulder-strewn and hilly with the only roads exposed to Syrian artillery located on high ground.

One Palestinian group said Israeli forces had pushed from a coastline beachhead south of Beirut to Aley, a Druze town in the mountains, but this was denied by the overall guerrilla command.

Syria claimed that Israeli aircraft early this morning bombed vehicles north of the northern Lebanese port city of Tripoli, near the Syrian border, killing 57 persons, including 14 Turks, and injuring 75 others.

Meanwhile, residents of West Beirut, rich and poor, worried nervously about a possible Israeli attack on the city.

The rich, who live along the coastline, were fearful their homes would be prime targets in any such attack. The poor are fleeing the shanty towns that have been hit heavily during the Israeli riads.

Already, the invasion has changed daily life. Schools and universities are shut. The capital's main power plant is in Israeli hands and electricity is limited to 4 1/2 hours a day on a rotating basis from one neighborhood to another. And with one of the country's two refineries also under Israeli control, gasoline is running short. So, too, is flour, and long lines formed early in the morning outside bakeries.

Beirutis have withdrawn large sums from banks, converting their once-sound Lebanese pound into gold and dollars lest the Israelis stay around and start using their own inflated currency inside Lebanon itself