B'nai B'rith International, the largest Jewish service organization in the United States, yesterday gave President Reagan's controversial Mideast peace plan a major boost by calling the initiative "worthy of consideration" and "giving fresh momentum to the search for Middle East peace."

The declaration, released by B'nai B'rith International President Jack J. Spitzer, did not endorse Reagan's plan in its entirety. It noted that the organization has "specific questions and problems with the president's proposals . . . ."

Nonetheless, people prominent in the American Jewish community agreed last night that the statement is of what one called "potentially historic proportions" because it indicates that the administration initiative appears likely to gain significant backing among American Jews despite its rejection by Israeli Prime Minister Menachem Begin's government.

While announcing yesterday that withdrawal of the multinational peace-keeping force in Lebanon would begin Friday, Reagan again made clear that he would press on tenaciously with the peace initiative despite the formal rejection by Begin.

Although administration officials are reluctant to discuss their strategy publicly, a major part of it is aimed at winning significant support in the American Jewish community and using that support to help persuade the Israeli public to consider the plan seriously.

B'nai B'rith has about 400,000 members in the United States. Earlier this week Thomas A. Dine, executive director of the American Israel Public Affairs Committee, the leading pro-Israel lobbying group, also praised the main thrust of the president's initiative during an interview with The New York Times.

Noting that the plan calls for granting eventual self rule to the West Bank and Gaza Strip "in association with Jordan," Dine stressed that the next move is up to Jordan's King Hussein.

After a meeting last week between Secretary of State George P. Shultz and leaders of major American Jewish organizations, there were indications that the American Jewish community is divided about how to respond to Reagan's plan.

The group did not reject the plan or criticize any of its specific parts, saying only that it had questions about whether the United States should abandon its neutral mediating role in the Middle East conflict and espouse a specific solution.

The B'nai B'rith statement was made public by Spitzer after a meeting here of the organization's public affairs advisory committee. It said:

"B'nai B'rith commends President Reagan for putting forth a Middle East peace plan which combines a U.S. commitment to secure and defensible borders for Israel with a formula designed to draw in new Arab negotiating partners.

"The president's plan is worthy of consideration not only because it recognizes the danger and impracticality of an independent Palestinian state but because it asks Jordan to take responsibility for negotiating directly with Israel on the future of the West Bank and Gaza Strip.

"While we have specific questions and problems with the president's proposals, we believe they are presented in a sincere spirit of finding common ground for involving all parties to the conflict in the negotiating process and giving fresh momentum to the search for Middle East peace."

In announcing the withdrawal plans, Reagan said "the U.S. Marine contingent should be among the first to leave." His remarks on pressing the peace initiative came as Begin vowed that the Israeli-occupied West Bank of the Jordan River will be a Jewish rather than Palestinian homeland forever.

"My own personal reaction," Reagan said confidently, "is that because I stressed negotiations as the settlement to many of these troublesome issues there, I think that we have to understand sometimes that, maybe, positions are being staked out with these negotiations in mind."

Reagan's brief remarks to reporters on the South Lawn of the White House followed a 30-minute afternoon meeting with Philip C. Habib, his special envoy to the Middle East.

The two met to discuss the peace initiative and current situation in Lebanon.

The president obviously was pleased that the Marines' deployment two weeks ago had been accomplished with no reports of major casualties and that they would be withdrawn ahead of schedule. Reagan said Habib had told him the crisis in West Beirut could not have been resolved peacefully without the multinational force.

The Marines' deployment marked the second time in 25 years that they had been sent to Lebanon on a peace-keeping mission. Initially, Reagan had promised that they would be withdrawn within 30 days. Friday would be their 16th day there.

France and Italy also provided troops for the peace-keeping force, which patrolled city streets and oversaw the departure of Palestine Liberation Organization fighters from their West Beirut sanctuaries.

Reagan, announcing formation of an administration interagency steering group on Lebanon, indicated that continuing peace efforts there would be handled by Morris Draper, deputy assistant secretary of state for Near Eastern and South Asian Affairs and Habib's right-hand man in negotiations leading to evacuation of Israeli-surrounded PLO forces from West Beirut.

Draper will be returning to Lebanon in a few days, Reagan indicated.

Not announced was what role Habib would play. Habib said he intends to return to Lebanon for the inauguration Sept. 24 of President-elect Bashir Gemayel, but said that beyond that, "My position is that I'm going to go take a vacation, and then I'm at the president's disposal, as always."

Earlier yesterday deputy press secretary Larry Speakes expressed White House concern about reports that Israeli warplanes had launched a new attack in Lebanon, destroying Syrian antiaircraft missiles in the Bekaa Valley, east of Beirut.

"We're extremely concerned about the reports of renewed military activity in the Bekaa Valley," he said. "We reiterate our strong view that all involved must act with the utmost restraint. Such incidents serve to re-emphasize the need for withdrawal of all our foreign forces from Lebanon."

Habib said removal of all outside forces from Lebanon remains a U.S. objective.

"As a matter of fact, it's an imperative," he said. "If Lebanese sovereignty is going to be re-established, it's imperative that external military forces leave Lebanon."

Rejecting the idea that he had achieved "success" in Lebanon -- he prefers the word "progress" -- Habib said that he nonetheless believes that "there is a fair, good chance that we can see a sovereign, integral, free pluralistic Lebanon . . . ."

Asked about his dealings with Begin, Habib said:

"He's like any elected head of a democratic government. He pursues the policies of his country with vigor, with great, great, I might say, intellectual capability. He's got a remarkable memory, never lets me forget a thing I've ever said before.

"But then I kind of pocket a few things he's said to me also. So we're rather even on that score.

"He speaks for a democratic country," Habib said. "He represents the interest of his country as he sees them."