The Lebanese government today approved a U.S.-mediated plan for withdrawal of Israeli forces from Lebanon, and Secretary of State George P. Shultz immediately flew here in hope of winning Israel's consent to the draft agreement.

Shultz refused to predict whether his week-long shuttle diplomacy was on the verge of success. But the mood among U.S. officials accompanying the secretary clearly was optimistic, and there was expectation that the drive to win a Lebanese-Israeli accord might be concluded when the Israeli Cabinet meets Friday to consider the matter.

Shultz also announced that he will go to Damascus Saturday to discuss the agreement with Syrian President Hafez Assad. The Israelis have made clear that they will not pull their forces out of Lebanon unless Syrian and Palestine Liberation Organization forces in that country also withdraw, and there has been considerable skepticism in Israel about whether Assad would cooperate.

Shultz said during the flight here, "Obviously, the Israelis are not going to withdraw if there is no indication of Syrian withdrawal . . . . But this is a negotiation between Israel and Lebanon, and what the Syrian position is, is a separate thing."

The secretary also said that he hopes to visit Jordan and Saudi Arabia, probably on Friday, before leaving the region. But, he stressed, the possibility of doing that will depend on what happens between Israel and Lebanon.

Regarding the plan approved by the Lebanese government, one senior U.S. official said, "I can't see how either side can say no to this and then defend their position."

When reporters aboard Shultz's plane asked whether an agreement seemed closer, the secretary replied: "Well, sure. We have a clear, very forthcoming position from the government of Lebanon. Now we'll see what Israel says."

After arriving here in the late afternoon, the secretary plunged into new talks with Israeli Prime Minister Menachem Begin, Foreign Minister Yitzhak Shamir and Defense Minister Moshe Arens. U.S. officials said late tonight that the Israelis want to study various points in the draft more closely and said that they would be back in touch with Shultz Thursday.

Following a meeting of more than an hour with Shultz, Begin told reporters:

"I have to say again the matters are very serious, very important . . . . The Cabinet, of course, is entitled and empowered to take such a decision. My two colleagues Shamir and Arens and I myself cannot do so."

Details about what was contained in the draft agreement carried by Shultz remained secret. But U.S. sources said it was "an amalgam" of proposals given by Israel to Shultz earlier in the week, Lebanese ideas that the secretary believes are acceptable to Israel and "bridging compromises" suggested by Shultz to cover remaining points of dispute.

The sources, who asked not to be identified, said the main emphasis in this latest draft is on resolving disagreements about the security arrangements in southern Lebanon that Israel insists are required to protect its northern border against future terrorist attacks.

There has been agreement on the principle of joint Israeli-Lebanese supervisory teams to ensure that the arrangements are being carried out. But Lebanon had balked at what it contends are Israeli plans to use the teams as a cover to maintain a residual presence inside Lebanon.

Before Shultz went to Beirut on Tuesday, Begin is believed to have made substantial concessions about the number and powers of the Israeli soldiers that would be included in these teams. Those concessions are understood to be the basis for the formulations about the supervisory teams in the draft brought back by Shultz today.

Shultz also is understood to have worked out a proposal for solving the dispute over the future status of Saad Haddad, a renegade Lebanese Army major who heads a militia allied with Israel in southern Lebanon.

According to the sources, the tentative plan, which would be implemented quietly outside the framework of a formal agreement, would integrate Haddad's forces into the Lebanese Army and give him a "substantial military role in the south," but he would not command the Lebanese forces there as the Israelis would like.

The Lebanese are understood to believe that the plan that they accepted today has a good chance of passing muster with Syria. Foreign Minister Elie Salem, standing beside Shultz in Beirut, said: "We are confident that what is in the higher interests of Lebanon will be supported by Syria."

On Monday, Assad warned Salem that he will not withdraw his troops if Lebanon makes what Syria regards as unacceptable concessions on an Israeli presence in the south.

In private, U.S. officials acknowledged that they do not know what Syria will do. But, they added, Shultz is working on the assumption that if Israel and Lebanon reach agreement, there will be pressure on Syria from other Arab governments and from world opinion not to block a withdrawal of all foreign forces from Lebanon.

U.S. officials also are known to hope that the possibility of getting Syria--which is engaged in a massive, Soviet-assisted military buildup--out of Lebanon will be a powerful spur for Israel to accept the proposed agreement.