Indications in Washington that the Reagan administration was modifying its apparent support of Israel's campaign in Lebanon drew no public reaction here tonight, as government spokesmen in quick succession acknowledged that a cease-fire had been agreed to and expressed "deep regret" over the resignation of Secretary of State Alexander M. Haig Jr.

The cease-fire agreement, after a day of heavy bombing in Beirut, was not officially announced, although Israeli military sources confirmed it in response to reports from abroad.

But Defense Minister Ariel Sharon, in an interview broadcast on Israeli television at 8 p.m. local time (2 p.m. EDT) just before the cease-fire was to begin, did not mention it. He declared that the Palestinian guerrilla movement had been broken and that Israel's overnight capture of a stretch of highway leading from Beirut to Damascus has made the siege of Beirut unassailable.

During a tour of the Beirut front earlier in the day, Sharon made comments that were seen as an indication of resistance to pressures for a cease-fire.

"The military and political campaign that we have been conducting now in order to eliminate the PLO terrorist bases in Lebanon has by now become a very great achievement for the security and peace of Israel, I would say, for an immediate range and the long-range," he said. "Therefore, all of us should keep cool--we must show patience--we should not be affected by any political provocations, internal or external, that exist and that may threaten our military and political achievements."

Throughout the campaign in Lebanon, the hard-driving defense minister has been reported pushing Prime Minister Menachem Begin and his Cabinet for a far-reaching military attack on the Palestine Liberation Organization guerrilla establishment, seeking to crush its potential permanently. It was unclear whether the official silence on the cease-fire reflected disagreement between Sharon and others in the Israeli Cabinet about the wisdom of bowing to reported pressure from Washington.

The cease-fire is the the latest in a series between Israeli forces and their opponents in Lebanon. Previous agreements have produced only brief respites in nearly three weeks of fighting that has left Israeli forces surrounding what is left of the PLO in the predominantly Moslem neighborhoods and refugee camps of West Beirut.

Most Israeli officials remained at home in observance of the Sabbath tonight, and there were indications that many of them were not aware of Haig's resignation until official statements were made.

Since Begin's return from Washington on Wednesday, Israeli government officials have given the impression that they enjoyed clear-cut backing for the Lebanon invasion, despite President Reagan's public reservations about civilian casualties and U.S. admonitions against an all-out ground assault on Beirut. Cabinet ministers reportedly were led to expect only mild protests from Washington for the heavy bombardment of the last few days, providing the aerial and artillery assault did not mushroom into a storming of the city likely to cause even heavier civilian casualties.

The conclusion about U.S. support for the operation has flowed from a noticeable lack of public criticism before and during Begin's visit to Washington earlier this week and, according to reports from diplomats and Israeli officials, private talks between Begin and President Reagan.

"If the president of the United States comes today," Foreign Minister Yitzhak Shamir said in a radio talk before the cease-fire, "and in his meeting with the prime minister he says that it is impossible to conduct terrorist and other attacks on Israel from the north, and if he says today that one must fight and struggle for the removal of all foreign forces from Lebanon, that means that the president of the United States identifies with the aims of the military campaign of the state of Israel."

The perception of U.S. support has seemed to provide a framework for the intense shelling and bombing of Beirut of the past few days in which Israel can badly damage the remaining guerrillas and still abide by Begin's reported pledge in Washington to avoid a ground assault on the besieged areas.

Despite the statements in Washington that Reagan urged Begin to refrain from such an attack, reports emanating from a Cabinet meeting chaired by Begin yesterday said the Israeli leader came away from his meetings with Reagan and Haig with the impression the U.S. government would not raise strong objections about air and artillery attacks on Palestinian guerrillas in Beirut as long as they fall short of an all-out storming of the capital.

Observers noted that Begin's return to Israel was followed by intensified shelling and bombing of targets within West Beirut, including the heavily populated Corniche Mazraa area where PLO groups maintain a number of offices alongside Lebanese apartment houses.

Amnon Rubinstein, a member of parliament from the opposition Shinui Party, said the U.S. decision to close its embassy in West Beirut was interpreted in the Israeli government and public as a sign Washington had decided to look the other way while Israeli forces pummel the PLO in what has been described here as psychological pressure to induce the guerrillas to surrender.

A high government official said Israeli intelligence services reported that PLO leader Yasser Arafat and other PLO leaders disagree strongly over what to do, with more radical guerrilla leaders urging a fight to the finish, and others, presumably including Arafat, suggesting that surrender is necessary to avoid destruction of West Beirut.