U.S. presidential envoy Philip C. Habib was reported today to be confident of concluding an agreement by Sunday for the departure of Palestinian guerrillas from besieged West Beirut.

Only sporadic shelling was reported in the Beirut area today, and Israel eased its blockade of the city somewhat. It allowed supplies of much-needed blood and plasma as well as some food to enter West Beirut, where 6,000 Palestinian guerrillas are trapped along with tens of thousands of civilians.

Habib was reported to have proposed a pragmatic solution to a key problem of when an international force, including U.S., French and perhaps other foreign troops, would enter West Beirut to supervise withdrawal of the guerrillas--a key Israeli demand for a negotiated peace.

Habib is also said to have suggested that all three related moves--Palestinian evacuation, token Israeli withdrawal and the international force's arrival--take place simultaneously. U.S. 6th Fleet vessels carrying about 1,800 Marines were reported to be 40 miles off the Lebanese coast.

As a further assurance of his understanding of Palestinian sensitivities, Habib was said to have promised to make both the Christian militia and the Israelis "invisible" as the guerrillas moved along the 18 miles of Israeli-controlled road on the Beirut-to-Damascus highway before they reached Syrian lines on their way to exile in Syria.

He told visitors he felt there was no imminent threat of an Israeli attack against the predominantly Moslem western sector.

But Habib--who later conferred again at the U.S. ambassador's residence with Israeli Defense Minister Ariel Sharon and David Kimche, the director general of Israel's Foreign Ministry--was said to feel it was essential to press ahead and conclude the negotiations in the shortest possible time.

Habib's reported optimism--which coincided with a similar mood in Israel from Prime Minister Menachem Begin on down--contrasted with PLO insistence that nothing had been settled and that time was on its side.

"The Americans are telling lies," a ranking Palestinian official said, "spreading exaggerated news stories and disinformation" hoping to stampede the guerrillas into concessions.

But Saeb Salam, the veteran former Lebanese prime minister who has mediated between Americans and Palestinians, made light of such Palestinian claims.

He insisted that the Palestine Liberation Organization had dropped its insistence on a U.N. blessing for an international force, including U.S. Marines, to supervise the Palestinian evacuation. He also confirmed American and Israeli reports that the guerrillas were now willing to travel by land to Syria.

The Palestinians have refused to leave by sea, especially on U.S. ships, associated in their minds with American military support for Israel.

Salam told reporters that only at 3 a.m. had he persuaded the guerrillas to sit down and discuss in detail a number of vexing problems.

Among those he listed were what to do with the some 1,000 non-Palestinian volunteers--mostly from other Arab countries and Europe--who are PLO fighters, and the perhaps 30,000 to 40,000 relatives of the guerrillas who do leave and the families of the 6,000 guerrillas now detained by Israel without formal prisoner-of-war status.

Despite such diplomatic problems, only minor noontime artillery fire disturbed the new--and sixth--cease-fire decreed by Israel since its troops invaded Lebanon June 6.

Even the Israeli blockade eased as the International Committee of the Red Cross was able for the first time since Saturday to bring in ambulances, medicine, desperately needed blood, and hospital equipment.

Some trucks carrying food were allowed into West Beirut for the first time but others were barred.

In East Beirut, the Israeli Army spokesman, Col. Paul Kedar, announced the withdrawal as of this morning from the Galerie Semaan, one of the three crossing points between West Beirut and the predominantly Christian eastern side of the capital. Kedar, according to Washington Post correspondent Leon Dash, said the policing of the crossing would be done now by Lebanese Christian forces.

But Lebanese and Palestinians in West Beirut insisted that Israel had not carried out a promise to turn responsibility over to the Lebanese Army.

Wazzan, who lives in West Beirut, accordingly once again refused to cross Israeli lines. He thus did not confer with President Elias Sarkis or Foreign Minister Fuad Boutros on a wide range of negotiating problems involving the Palestinians, whose views he presents to the other members of the government.

However, traffic in and out of the Galerie Semaan crossing point was permitted on and off during the day although few drivers chose to slalom their way through the mines the Palestinians have laid on the road.

Correspondent Leon Dash reported from East Beirut:

There was slight sporadic shelling by Israel of Beirut's southeastern suburb of Burj Barajneh, the site of major Palestinian settlements.

Israeli Army spokesman Kedar said that in shelling between the Israelis and Palestinian forces since Tuesday, five Israeli soldiers have been killed and four wounded.

The Red Cross has a warehouse crammed with food in the Lebanese port of Jounieh that the Israelis have so far not allowed them to take into West Beirut, officials said.

A Catholic Relief Services spokesman said that his organization was allowed to take food into West Beirut today through the Israel-manned military roadblocks in the port for the first time since last weekend.