Israel allowed water and electrical service to be resumed to besieged West Beirut today but a new exchange of heavy shelling between Israeli forces and trapped Palestinians left sections of the city in flames.

The Israelis also lifted their blockade of West Beirut long enough to allow one 23-ton shipment of foodstuffs provided by a foreign charity to enter but otherwise maintained a tight seal on all three entrances into the predominantly Moslem half of the Lebanese capital, preventing food, fuel and medicine from entering. No explanation was provided for the exception.

Meanwhile, formal support from the Lebanese government grew for the U.S. proposal for an international force. Prime Minister Shafiq Wazzan told newsmen the Lebanese government was studying the U.S. plan and might reply "in a day or two."

Foreign Minister Fuad Boutros, a leading Christian member of the government previously opposed to such ideas, said the Lebanese government was not opposed in principle to "military internationalization" by Washington if all Lebanese parties agreed. Karim Pakradouni, a spokesman for the Christian Maronite militias, withdrew his organization's opposition to any foreign military presence in Lebanon for a "transitional period."

A ranking PLO official today demanded that any international force have prior United Nations' approval as an added guarantee against feared Israeli attack.

The PLO officially is still demanding a cease-fire to be followed by a disengagement of forces guaranteed by an international force. The force would be deployed before the guerrillas or their leaders left Lebanon. But Israel and some Lebanese fear that any such disengagement of the combatants would allow the PLO to delay its departure almost indefinitely.

PLO officers have said that between eight and 12 weeks would be required for an evacuation if any agreement were reached.

There was a flurry of activity among Palestinians and their supporters this afternoon over a reported message from U.S. special envoy Philip C. Habib. It apparently relayed an Israeli position that U.S. participation in an international force could begin only after the guerrillas had left their West Beirut redoubt.

This interpretation apppeared to contradict President Reagan's definition of the role of such an international force as a protector of the guerrillas during an evacuation, and it was unclear whether a message from Habib might have been misconstrued. Nevertheless, Wazzan called an afternoon meeting with top Lebanese Moslem leaders and Palestinian chief Yasser Arafat and his principal lieutenants to deal with the situation.

Wazzan later said only that every day brought "good progress" but also "new obstacles" in the negotiations.

Another participant in the meeting said the new position, if accepted, would amount to "suicide" for the Palestinians.

The whole point of such an international force, he said, was to protect both the departing guerrillas and the remaining Palestinian civilians from any Israeli advance into the predominantly Moslem western sector of the divided city.

Further complicating the situation were reports that the Palestinians now preferred to leave by land rather than by sea under the protection of the U.S. 6th Fleet, which the United States has offered.

One Lebanese Moslem leader, who asked to remain anonymous, said he had asked Habib for a personal guarantee to fly out George Habash, leader of the militant Popular Front for the Liberation of Palestine, who is a bete noire of the Israelis.

In today's military action, which violated yet another cease-fire negotiated yesterday by Habib, the fifth since the Israelis invaded June 6, Israeli and Palestinian gunners again traded artillery fire late this afternoon. Israeli gunboats offshore joined in again pounding Palestinian targets in the southern suburbs. But the shelling stopped soon after dark.

Overnight the Soviet Embassy compound and its nearby six-story trade mission were hit by at least seven rounds that caused extensive damage, but no casualties. The Soviets charged the Israelis also damaged the embassy last month. Israel insisted it was not responsible for that attack.

An Israeli military official said this afternoon's artillery fire on Palestinian positions in the southern suburbs was in retaliation for the deaths of five Israeli soldiers last night.

He acknowledged that the Israelis had begun this round of fighting, saying, "We started it this time because they started it yesterday."

From a vantage point on a hilltop overlooking the southern suburbs, there did not appear to be any significant fire from the Palestinians in return. Most of the suburbs, located north and east of the Beirut airport, appear to have already been evacuated of their Palestinian and Lebanese Shiite residents.

Israeli tanks positioned on a plateau near the Lebanese Arab University could be seen firing at close range into buildings in the suburb of Hayy es Sollom. Soviet-made Katyusha rockets captured from the Palestinians were fired by Israelis from positions south of the airport, starting fires in the area of the Burj Brajneh Palestinian camp.

U.S.-supplied 175-mm, self-propelled guns joined the shelling, pouring fire onto targets about half a mile below the hilltop Maronite Christian St. Antoine College, where Israeli troops are bivouacked.

Palestinian sources said today that they have received nothing of a positive or substantive nature in reply to Arafat's signed document of Saturday night in which he is reported to have agreed to a withdrawal under certain conditions. This has heightened their present state of determined but desperate attitude, increasing their suspicions of the United States and of the current indirect form of negotiations.

In this atmosphere, the only channel the Palestinians appear to trust is via the French, which goes from Washington to Paris, then to PLO representative Ibrahim Souss for transmission to Arafat. The Saudis, the Egyptians and all other channels are full of "static" and deemed unreliable, one official said.

The official also expressed concern that the Lebanese, the Egyptians, the Saudis and others seem to be muddying the waters with proposals reflecting their own interests so much that "maybe the Americans are getting the wrong impression."