A senior Israeli government source said today that the "leakage" in Washington of a new American proposal that Israel unilaterally set a date for withdrawal from southern Lebanon had damaged current U.S. efforts to arrange for a simultaneous Syrian pullout from eastern Lebanon.

The source said the reports probably had given the Syrians the impression that the U.S. position on Lebanon was changing and that Washington was about to press Israel for a unilateral withdrawal, leaving Syria free to remain in Lebanon.

"I cannot deny damage has been done," the source said. "Whoever leaked it did cause damage."

The source, who cannot be identified under the ground rules of the briefing, said that international politial pressure on Syria to evacuate its 40,000 to 50,000 troops stationed in Lebanon's Bekaa Valley is essential to changing the present Syrian refusal to budge.

"If they believe there is hesitation, it is not a useful thing," he said. "How can one set a timetable? The Syrians will then just have to sit and wait until we withdraw."

Asked whether he thought U.S.-Israeli relations also had been damaged by the leak, the source said Israel believed "there is no change in the U.S. position."

The official's comments underscored the general irritation felt within the Israeli government over the reports. They appeared on the front pages of yesterday's editions of The Washington Post and The New York Times and quoted unidentified sources within the administation as saying the United States had advanced the idea as a possible way of putting pressure on Syria to follow suit and abandon its present total opposition to the Lebanese-Israeli withdrawal agreement.

But the Israelis clearly believe the leak has complicated the process of their "redeployment" in southern Lebanon because the Syrians may take it as indicating a weakening of Israeli resolve to stick it out there.

The Israeli Cabinet met today to discuss U.S. envoy Philip Habib's meetings last week with Prime Minister Menachem Begin and Lebanese leaders, but a spokesman said the issue of an Israeli "redeployment," the euphemism here for a unilateral partial withdrawal, had not been fully discussed, "although the question was raised."

The spokesman, Cabinet Secretary Dan Meridor, said there "probably" would be a discussion of the redeployment issue within the government before U.S. Secretary of State George P. Shultz arrives here later this week, but he said no decision had been made on whether to hold a special Cabinet session on the matter.

Meridor also made it clear that Israel rejects any idea of setting a timetable for a unilateral Israeli withdrawal from Lebanon.

"I don't see any point of one side determining a deadline or a date for a withdrawal if it is unconditional and not attached to the withdrawal of the other side," he said after the Cabinet's regular weekly meeting.

"Our position is the withdrawal should be simultaneous, Syrian and Israeli, after the PLO Palestine Liberation Organization has left. I don't see any use of setting a deadline now."

It is still not clear here at what level the reported U.S. idea of a timetable for a unilateral Israeli withdrawal was brought up. The senior Israeli source said "there might have been" some discussion between ambassadors of the two countries but it was unclear to whom he was referring and he seemed to suggest that contrary to reports coming from Washington it had not been suggested by Habib himself.

He seemed to be seeking to contain whatever damage the reports had already done in Israeli eyes and to be eager to downplay its importance.

The Israeli government is under increasing pressure from its own public to take steps to reduce the number of casualties being inflicted by guerrilla hit-and-run attacks on the roughly 20,000 Israeli troops in southern Lebanon, and the Army has drawn up contingency plans for a partial withdrawal along one of three rivers near the coastal city of Sidon, 25 miles south of Beirut.

The senior government official again stressed Israeli determination to make up its own mind free of any U.S. pressure. But he said Israel hoped there could be coordination among the three governments, including Lebanon's, over any Israeli redeployment and measures to prevent Syrian troops or PLO guerrillas from filling the vacuum.