Israel reacted cautiously today to reports of the Palestine Liberation Organization's willingness to leave Beirut and indicated it will ignore a U.N. Security Council resolution calling on it to allow the free flow of food, water and electricity to the besieged city.

Any report that the PLO is now ready to leave West Beirut peacefully, including the Arab League's proposal in Jeddah, Saudi Arabia, of a six-point plan for the evacuation of the Palestinian guerrillas, "we take with more than a grain of salt," an Israeli official said.

The official made clear that Israel will wait until it hears directly from U.S. special envoy Philip C. Habib before it will even consider altering its deeply skeptical attitude about PLO intentions. As of midday, there was no such message from Habib, officials said.

Habib left here Wednesday for Beirut after reportedly promising Prime Minister Menachem Begin that he would seek an "unequivocal commitment" by today that the PLO forces are willing to leave Beirut. Habib was said here to have been given a "last chance" to resolve the crisis peacefully, a point that was underscored by the break today in the latest cease-fire arranged by the American diplomat.

Late this afternoon, Israeli military officials announced that their forces had begun to return PLO fire around Beirut. The Israelis, who accused the Palestinian guerrillas of violating the cease-fire both yesterday and today, added:

"The Israeli Defense Force does not consider itself obligated to maintain or observe a unilateral cease-fire."

Meanwhile, Begin reinforced the Israeli hard line and the sense that Habib's mission is nearing its conclusion one way or another. According to Israeli radio, Begin told a closed meeting of the Knesset (parliament) foreign affairs and defense committee that Israel did not yet have the "unequivocal commitment" it is seeking and warned that if the PLO did not leave Beirut soon, "Israel will step up the pressure and possibly launch a military operation."

Begin's office also announced that the prime minister's trip to Zaire, scheduled to begin Sunday, was being postponed at the request of Zairian President Mobutu Sese Seko. An official in Begin's office acknowledged that the cancellation "cries out for a connection" with the Lebanon crisis but denied that this is the case.

Begin asked that the visit, which is important to Israel because of the rupture in its relations with most black African nations following the 1973 Arab-Israeli war, be rescheduled for between Aug. 16 and 24, suggesting that he does not want to be away from Jerusalem in the next critical weeks.

Israeli officials shrugged off the U.N. Security Council resolution, which was sponsored by Spain and adopted without the participation of the United States, calling for an immediate relaxation of the siege of Beirut to allow food and other supplies to enter the western sector on a regular basis. Israel has intermittently cut off supplies of food, water and electricity to West Beirut as a form of pressure on the PLO forces trapped there.

Asked about the U.N. resolution as he prepared to leave today for Washington, Israeli Foreign Minister Yitzhak Shamir told reporters in Tel Aviv that while he had not seen the full text of the document, "I am sure it will not change the situation."

An Israeli official who briefed foreign correspondents here responded to a question about the flow of supplies to Beirut by reading from a press release issued by the U.N. Relief and Works Agency that accused the PLO of preventing supplies from reaching refugees in West Beirut and southern Lebanon for the past 10 days.

Pressed on the issue, the official said that Israel is "trying in all ways to convince the PLO that its only option is to leave Beirut and Lebanon. What you are referring to is part of that process."

The official also charged that those supplies the Israelis have allowed into West Beirut had been taken by the PLO guerrillas and had not reached the civilian population. Asked to substantiate the charge, the official suggested only that Israel has ways of knowing what is going on in the city.

It was clear from the comments of Israeli officials that Habib's attitude will be critical in overcoming their skepticism of PLO intentions. As part of the commitment it is seeking, Israel wants "at least a timetable" for a PLO evacuation of Beirut, one official said.

"The commitment has to be clear to Philip Habib, who is not convinced of the willingness of the PLO to leave, and we have to be convinced of its sincerity," he said.

The official said Israel viewed the Arab League plan as a "form of recommendation" by the Arab countries to the PLO. "We don't know of any commitment to execute" the PLO withdrawal as proposed by the Arab League, he said.