President Reagan, using language that implied the threat of an embargo on U.S. arms shipments to Israel, warned Prime Minister Menachem Begin yesterday that Israel's assault against West Beirut was a "disproportionate" move and raises serious questions about whether the Jewish state is using American weapons for "legitimate self-defense."

That warning is understood to have been contained in a message sent by the president to Begin in an effort to halt the latest Israeli attack against Palestine Liberation Organization fighters besieged in West Beirut.

In it, Reagan charged that the Israeli action threatens to damage beyond any hope of salvage the efforts of the president's special envoy, Philip C. Habib, to negotiate the PLO's withdrawal from Beirut.

The president also demanded immediate restoration of a cease-fire, but administration officials said that, as of last night, no reply had been received from Begin.

The message sent to the prime minister, while known to be extremely tough in tone and reflective of great anger within the administration, actually was toned down somewhat from language that was considered originally.

For example, while it contained a clear implication that Israel's military supply relationship with this country could be jeopardized, it did not make the threat explicit.

Similarly, while the message is known to have referred to the total range of U.S.-Israeli relations, it did not contain, in the form sent to Begin, language advocated by some administration officials that would have implied a possible reassessment of U.S. non-military aid and diplomatic support for Israel.

Whether Reagan will be forced to consider taking such steps was still unclear last night. Administration sources said further decisions would depend on whether Israel cooperates in restoring the cease-fire in a way that permits Habib to resume his efforts with a reasonable chance of success.

U.S. officials are known to believe that Habib finally had started to make some headway in his six-week-old attempt to resolve the crisis without further massive bloodshed.

Reagan appealed for Israeli restraint in a meeting here Monday with Israeli Foreign Minister Yitzhak Shamir, but the latest Israeli assault was launched even before Shamir's departure from Washington yesterday.

Despite the intense anger directed at Begin behind the scenes, the president sought publicly to maintain a balanced approach toward the Lebanon crisis. The White House released a statement from Reagan that was described as giving the gist of messages he had sent to Begin and to Arab governments that have influence with the PLO.

In the statement, he said it is an "absolute necessity" that Israel halt its attack. But he also called on the PLO to leave Lebanon without further delay.

One apparent sign of Reagan's anger was provided by House Minority Leader Robert H. Michel (R-Ill.), who said, after a White House meeting, that the president appeared "distressed," and added that something "troubles him a great deal."

Adding to Reagan's concerns were signs of mounting pressure from the Arab side of the Mideast conflict for the United States to restrain Israel.

Saudi Arabia's King Fahd, one of the most important U.S. allies in the Arab world, was described by Saudi news agencies as having taken the unusual step of telephoning Reagan directly yesterday and warning him of the urgent need to take whatever measures are necessary to bring Israel into line.

At the root of the dilemma that has confronted the administration since Israel launched its invasion of Lebanon on June 6 has been U.S. agreement with the basic Israeli goal of forcing the PLO's leaders and guerrilla fighters out of Lebanon. However, the United States has been counting on Habib to accomplish that objective through diplomacy that will avert an all-out Israeli attack against the PLO fighters under siege in West Beirut.

The Israeli view, which was restated by Shamir in his talks here, is that it is unrealistic to expect the PLO to depart peacefully unless it is reminded constantly that it faces possible annihilation. Israeli officials have said repeatedly that while they regard a negotiated settlement as the preferred solution, they do not intend to surrender their military option, and they consider the U.S. position that negotiations offer the only road to a settlement as an unrealistic attempt to have things both ways.

In diplomatic circles, there is a growing feeling that the Habib mission is fast running out of time and steam. Many diplomats believe Reagan soon will have to make a choice between trying to force Israel to settle for something less than the total destruction of the PLO in Lebanon or condoning the bloodshed and destruction of a full-scale Israeli assault on the city.

However, as his statement yesterday made clear, the president still seems hopeful that such a choice can be avoided if he is able to buy more time for Habib. The statement said in part:

"I had made clear to the Israeli government, in my meeting with Foreign Minister Shamir, that the U.S. placed great importance on the sustained maintenance of a cease-fire . . . . This is a necessary first step toward our goal of restoring the authority of the government of Lebanon, a goal Ambassador Habib is earnestly working toward . . . .

"Through governments which have direct contact with the PLO, I have further expressed my strong conviction that the PLO must not delay further its withdrawal from Lebanon. At the same time, I have expressed to the government of Israel the absolute necessity of reestablishing and maintaining a strict cease-fire in place so that this matter can be promptly resolved."

The Israelis, with an obvious eye on U.S. public opinion, sought yesterday to deflect protests triggered by its advance on West Beirut. At a news conference, the Israeli military attache here, Maj. Gen. Menachem Meron, contended the move was not a full-scale invasion but a limited response aimed at stopping the PLO from firing at Israeli troops.

Shamir, speaking to a meeting of American-Jewish leaders in New York, said Israel was willing to reinstitute the cease-fire "from this moment" if the PLO stopped shooting. He added: "Let them stop firing, and there will be no more firing in Lebanon until Philip Habib will come and say, 'I did all in my capacities, all that I could, and this is the result.' "

Last year, the United States temporarily held up deliveries of jet fighters to Israel after it bombed Iraq's nuclear reactor and again after it launched air strikes on Beirut. Reagan also suspended a U.S.-Israeli strategic cooperation agreement after Israel annexed the occupied Golan Heights. Last week, he suspended indefinitely the supply of U.S.-made cluster bombs.

However, as U.S. officials concede, none of these actions had much effect on Begin's aggressive approach to dealing with Israel's Arab enemies. The feeling in diplomatic circles is that the only sanctions capable of making an impact in the Lebanon crisis would have to be of a severity that might cause traumatic strains in the 34-year history of close ties between the two countries.