Israel increased pressure on besieged West Beirut today, inching troops forward under intermittent artillery fire in the southern suburbs and stationing an entire armored brigade near the divided capital's main crossing point.

The action came as the Israeli commander in Lebanon warned residents of West Beirut, recovering from yesterday's 14-hour Israeli bombardment, "to leave before it is too late."

The armored brigade, estimated at 40 tanks, 150 armored personnel carriers and 155-mm self-propelled howitzers and about 3,000 men, was positioned in broad daylight in the area of the national museum, where hundreds of cars laden with possessions waited to cross into the relative safety of East Beirut.

Military specialists here insisted that the brigade's presence was a diversion, but its placement so close to West Beirut raised fears that the Israelis were about to launch their much delayed all-out ground assault against the trapped Palestine Liberation Organization forces in the predominantly Moslem sector of the capital.

Some of the tanks were equipped with flails to explode the mines the Palestinians and their Syrian and Lebanese allies have laid on main access roads to delay any Israeli ground attack.

The Israeli commander, Brig. Gen. Amir Drori, in warning residents of West Beirut to leave, appeared to put little faith in the cease-fire--the ninth in the conflict--that ended yesterday's fighting. "We can do nothing for those who remain," Drori said in a message broadcast by state-run Radio Lebanon.

Ever since the Palestinians were surrounded in the capital seven weeks ago, the Israeli forces have been thought capable of launching an armored thrust from the museum area to the Mediterranean to cut the PLO forces in two.

Such an operation, combined with a drive up the coastal road from the south and perhaps another thrust from predominantly Christian East Beirut through the port area, could allow the Israelis to overrun the badly battered refugee camps in the southern suburbs. In the port today, the Israelis stopped work and ordered dockworkers to go home.

According to that scenario, the Israelis then could turn their attention to the remaining guerrillas spread out in West Beirut proper, bringing the full weight of their superior fire power to bear against a foe trapped in an ever-shrinking defensive perimeter.

The Israelis, who Sunday completed their control of all but the northwest corner of the airport complex and moved several hundred yards farther north, today expanded their perimeter by taking Mreigi and parts of Hay Al Salloum, the suburbs adjacent to the airport.

Correspondents who visited the front reported that Israeli artillery in the hills to the east was firing on Palestinian positions due west of the airport road.

At some points the combatants were only 30 yards from each other, although in most cases they were separated by about 200 yards, the correspondents said.

There were no definitive casualty figures for yesterday's fighting, but dead and wounded were thought to be in the range of about 500.

A Palestinian official said his side's losses were "only 20 to 30" killed, a figure that struck some observers as low. The official said the Palestinians lost no heavy guns in the fighting.

Military specialists, however, are convinced the Israelis knocked out a number of heavy weapons, including some of the last remaining ones belonging to the dwindling Syrian forces in West Beirut.

Meanwhile, the Lebanese gendarmerie today published statistics covering Palestinian and Lebanese casualties in July. The list said that 963 persons were killed and 2,013 wounded in Beirut and the Eastern Bekaa Valley during the month and that most of the victims were civilians in Beirut.

Wafa, the Palestinian news agency, claimed the airport operation yesterday was "the most spectacular failure Israel has suffered since the war began on June 6."

Wafa reasoned that the main upshot of what Beirut newspapers called "Black Sunday" was "increasing international pressure to stop the bloodshed" and "increased United Nations involvement" in the conflict.

In the only local diplomatic activity of the day, U.S. special envoy Philip C. Habib conferred with a Lebanese general about details for a timetable dealing with the Palestinian guerrillas' eventual departure from Beirut and Lebanon.

In turn, the general conferred with Hani Hassan, a leading Palestinian negotiator, in preparation for a meeting tomorrow with a PLO military team.

After yesterday's attack, the overwhelming impression among Beirut's residents was that the Israelis in a single day had wrought almost as much destruction in some places as the various combatants had achieved in the 19-month civil war in 1975 and 1976.

Wafa claimed the Israelis had fired 185,000 projectiles--or three a second--and flown more than 300 sorties with 60 aircraft during the attack.

Newly collapsed houses and entire barracks bore the traces of repeated Israeli air strikes, while bomb craters and shell holes covered entire streets.

Oddly, some tanks and armored vehicles escaped destruction although apparently without cover.

Scores of buildings ranging from expensive modern apartment houses to more humble dwellings displayed shell holes or burned out walls.

Saeb Salam, the 77-year-old veteran former prime minister, made a tour through the city to inspect the damage, and said, "I am angry at America; I am angry at President Reagan" for tolerating the Israeli "destructiveness."

Toting up the destruction, which included schools, mosques, churches, hotels, hospitals, embassies, gendarmerie barracks, Salam said he was particularly disheartened by the Israelis' use of white phosphorus against the pine forest near the city's dividing line.

"They may be only trees," he said, but "those trees in the pine forest are 1,000 years old and are the pride of Beirut."

Near the forest, Amal Shamma, a young American-trained doctor showed visitors around the damage at Barbir Hospital. The bombardment scattered plaster around the sixth-floor blood bank and laboratories and damaged three rooms in the nurses' quarters below, she said.

"We've lost 75 percent of our personnel," she said, noting that within minutes of the cease-fire, relatives arrived to take away many remaining staffers.

Some staffers never reported for duty when the war began, and others fled when the first Israeli leaflets were dropped later that month warning Beirut residents to flee.