Israeli Prime Minister Menachem Begin told two U.S. senators today there is no Israeli deadline on efforts to find a way to evacuate peacefully Palestinian guerrillas and their leaders trapped in West Beirut.

Speaking to reporters after an hour-long meeting with Begin, Senators Christopher Dodd (D-Conn.) and Carl Levin (D-Mich.) said they were told there is "no ultimatum whatsoever in regard to West Beirut on the part of Israel."

It appeared from the remarks attributed to Begin by the two U.S. senators, and from a briefing by an Israeli official, that Israel has acceded for the time being to persistent pressure from the Reagan administration to allow Habib more time to pin down details of the PLO departure.

There were, nonetheless, indications today that Israel and the United States were still at odds over a proposal that the PLO be permitted to leave behind a small political mission and two 250-man military units under Lebanese Army command.

An Israeli general announced extended military service and preparations for Israeli invasion forces possibly to spend the winter in Lebanon, United Press International reported.

Deputy Chief of Staff Moshe Levy told military correspondents that compulsory service of soldiers completing their initial three-year term would be extended by three months to meet the demand for extra manpower and that the defense budget may be increased by $1 billion because of war-related expenditures.

Begin's comments as reported by the two senators followed a statement by U.S. Defense Secretary Caspar W. Weinberger that the Israeli Cabinet had placed a one-week deadline on the tortuous negotiations at its meeting last Sunday.

"Habib," Weinberger was quoted here as saying, "has until next Sunday to put together an agreement."

The two senators also said the Israeli prime minister is "very optimistic" about settling the still unresolved issue of conditions under which the PLO will leave West Beirut.

"The prime minister wants to give U.S. special envoy Philip Habib every opportunity possible to secure a peaceful resolution of the withdrawal of the PLO," remarked Dodd.

Despite these reports of official optimism, there were clear signs from other Israeli officials that deep suspicions remain about whether PLO leader Yasser Arafat is truly committed to carrying out the withdrawal in light of the interminable haggling over the means for accomplishing it.

"We don't know if they are leaving or not definitely," said on Israeli official briefing foreign correspondents here today. "They are coming up with various ideas, conditions and plans every day.

Adding to the confusion over the last 24 hours, he said, were new doubts among Israeli officials about whether Syria is really prepared to take some of the departing guerrillas on a permanent basis as it had earlier indicated, or is only willing now to serve as a temporary transit point for them.

He indicated that Israel is now convinced that Habib is staying on in Beirut after two weeks of negotiating with the various parties involved because there is not yet any precise agreement with the PLO.

"Had it their departure been so clear, Habib would have been done long ago," he said.

The official said the main purpose of the visit to Beirut today and yesterday of David Kimche, director general of the Foreign Ministry, was to drive home to Habib Israel's insistence that the PLO leave behind no military or political presence anywhere in Lebanon.

The two senators said Begin made clear the same point during his meeting them them today.

Reuter news service reported that Israeli Defense Minister Ariel Sharon had conferred Thursday with Habib in Beirut.

Israeli officials also reported that Begin had informed President Reagan that there was no danger of starvation or lack of water in besieged West Beirut, Reuter said.

The official insistence today on Israel's opposition to any remaining PLO political or military presence in Lebanon seemed partly aimed at countering reports in the Israeli press suggesting the government might yet relent on these two issues once a firm agreement has been reached on details of the guerrillas' evacuation.