Imposition of U.S. sanctions against Israel would be certain to backfire, producing an Israeli military reaction that could make the siege of Beirut "look like peanuts," a senior Israeli official warned today.

His suggestion that U.S. pressure would provoke an "unpredictable" Israeli response followed reports here that Prime Minister Menachem Begin has written a letter to President Reagan rejecting Reagan's call for an Israeli pullback from positions Israeli troops have seized in West Beirut. Details, Page A18.

In another development that indicated strong Israeli skepticism about reports from Beirut that U.S. special envoy Philip C. Habib was on the verge of concluding a deal that would lead to a Palestinian withdrawal, Voice of Israel radio reported today that the Israeli government would not accept a reported concession offered to Habib by Palestine Liberation Organization Chairman Yasser Arafat.

Arafat has reportedly agreed that the guerrillas will begin to withdraw simultaneously with thedeployment of a multinational force in West Beirut. This would enable the guerrillas to leave within two weeks, Habib has reportedly told Washington.

But the broadcast said Israel would not agree to a simultaneous withdrawal and deployment. Israel is insisting that the guerrillas move out first so thay they cannot use the multinational force as a screen behind which to stay in Beirut.

In the Israeli broadcast, David Kimche, director general of the Foreign Ministry, said that simultaneous PLO withdrawal and peace-keeping force arrival would spell a "tremendous defeat" for Israel, the Lebanese government and "the whole civilized world."

An Israeli Army spokesman, Michael Kevehazi, said he doubted that Israel could accept the point in the reported proposal that calls for an overland PLO withdrawal along the Beirut-Damascus highway, Washington Post correspondent Leon Dash reported from East Beirut.

"What is to prevent them from joining the Syrian forces once they have passed the Israeli lines at Sofar?" he asked. The Israeli and Syrian armies are facing each other along a cease-fire line at Sofar, 15 miles east of Beirut. Israel had indicated previously that it would not object to use of the Beirut-Damascus highway for withdrawal.

The Israeli official who predicted a strong Israeli reaction to possible American sanctions charged that American efforts to lessen Israeli military pressure on the Palestinian guerrillas trapped inside West Beirut are undermining the chances of a peaceful resolution of the Lebanon crisis. He said that those in the Reagan administration who favor sanctions against Israel seriously misunderstand Prime Minister Begin.

"It will have a contrary effect and America will lose all of its leverage," he said. "Then what Israel will do is unpredictable, but it could make Beirut look like peanuts."

The official, who was interviewed on condition that he not be identified, was clearly eager to deliver the warning to American leaders, including President Reagan, who have been expressing mounting anger and frustration over the latest military moves into West Beirut.

The official noted that previous American efforts to exert pressure on Israel produced only verbal responses, such as the tongue-lashing that Begin delivered to U.S. Ambassador Samuel W. Lewis late last year after the administration suspended the strategic cooperation agreement with Israel following the Israeli bombing of the Iraqi nuclear reactor near Baghdad.

"This time we are at war, and there is no doubt the reaction will be on the ground," he warned. "It will be very painful."

The official declined to specify what he meant by a reaction "on the ground," but he clearly meant to imply some form of military action, and he appeared to be alluding to the presence of Syrian forces in the Bekaa Valley in eastern Lebanon.

In addition to demanding the withdrawal of the Palestine Liberation Organization fighters from Lebanon, Israel has said its invasion forces will withdraw only when there has been an agreement that all foreign forces--including the Syrians--will leave the country.

At the same time that the warning on sanctions was being delivered, Israeli officials briefing foreign correspondents explicitly rejected Reagan's demand for a pullback by Israeli forces to the lines they held Aug. 1, before the latest incursion into key points inside the western sector of the city.

Arguing that the only hope for a peaceful withdrawal of the PLO forces lies in Israel's continued threat to exercise its "military option," the official said that such an Israeli withdrawal would mean the immediate collapse of the negotiations being conducted by U.S. special envoy Philip C. Habib.

This continuing Israeli attitude--that only military pressure will force the PLO to agree to a withdrawal--was put into practice again this afternoon in the skies above Beirut. Military officials announced that Israeli planes bombed Palestinian positions in the city in response to what they called "repeated violations" of the cease-fire by the PLO.

In contrast, the United States has been pressing Israel to exercise military restraint around Beirut in order to provide some "breathing space" for the Habib mission. This U.S. appeal was contained in a letter from Reagan to Begin and in this week's public declaration in Washington calling for the Israelis to pull back from their newly captured positions in West Beirut.

The Israeli Cabinet, meeting late into the night, yesterday indirectly rejected the American appeal, declaring in a statement that the deployment of Israeli forces in Beirut would be determined only after the PLO had left the city.

The same Cabinet statement bluntly rejected a United Nations Security Council resolution that called for the placement of observers around Beirut to monitor the cease-fire.

Today, officials said Israel remains willing to abide by a cease-fire, including one "in place" as called for by the United States. But they said they would do so only if the PLO also strictly adheres to the cease-fire.

Israeli sources said a similarly indirect rejection of the U.S. call for a pullback is contained in a letter Begin has sent to Reagan. Begin told the president that the recent Israeli military moves were prompted only by PLO provocations, the sources said.

In a letter drawn up during a 5-hour emergency meeting Thursday with the Israeli Cabinet, Begin told Reagan the security of Israel is the nation's top priority in the current crisis, the prestigious Ma'ariv daily newspaper said.

"The security of Israel and its citizens hangs in the balance," the newspaper quoted Begin as saying.