The Israeli Cabinet approved the first part of a three-stage troop withdrawal from Lebanon tonight and said the initial pullback will be accomplished within five weeks.

The Cabinet gave tentative approval -- subject to later votes -- to the remaining stages of the plan, which calls for the Israeli Army to withdraw to the border by about next fall.

The first stage is to be a pullback along the Lebanese coast, from the Army's current positions on the Awwali River to a new line between the Zahrani and Litani rivers south of Sidon, the main population center in southern Lebanon.

The second stage, tentatively planned for this spring, calls for a similar pullback in the eastern sector of southern Lebanon in which the Israeli Army would abandon positions that now put it within rifle range of Syrian soldiers and artillery range of Damascus. The new line in eastern Lebanon will be near Hasbayya, according to the Cabinet announcement.

The third stage, Israeli officials said, would be the pullback to the border, leaving a "buffer zone" six to 12 miles wide that would be controlled by the Israeli-backed South Lebanon Army and regularly patrolled by Israeli units.

The Cabinet approved the plan, drafted by Defense Minister Yitzhak Rabin and senior military officials, by a vote of 16 to 6, with all of the opposition coming from members of the right-wing Likud bloc in the national unity government. However, as part of a compromise agreement, the timing of the second and third stages of the planned withdrawal was made subject to future Cabinet votes, giving opponents of the plan a chance to scuttle it later.

Addressing a meeting of foreign correspondents a few hours before the Cabinet vote, Shimon Peres, the Labor Party prime minister of the unity government, said Israel still hopes that Syria and Lebanon will agree to security guarantees in southern Lebanon before the withdrawal plan is fully implemented. But Peres also made clear his determination to push for completion of the plan this year regardless of Syria's attitude or the continuing opposition within his own government to a complete withdrawal.

"Initially, we did not go into Lebanon to remain there forever," he said. "Well, if not forever, a time must come to decide. We feel the time has come. I do not see any reason to stay and wait and wait -- for what? The time has come to decide."

Peres said Israel's military delegation will return Thursday to the troop withdrawal talks with Lebanon that began Nov. 8 at the U.N. southern Lebanon headquarters in the border town of Naqura. But Peres added, "Unless there is a change in Lebanon's position -- which unfortunately depends completely on Syria's position -- I don't see that much will happen at Naqura."

In Washington, the State Department said: "We have consistently supported efforts to bring about the total withdrawal of all foreign forces from Lebanon, and we encourage the parties to continue their efforts to bring about a negotiated withdrawal as the best way to attain this objective. We continue to support the talks at Naqura as a means of achieving the arrangements necessary for a resolution of the problems in southern Lebanon."

Sources familiar with the Israeli-Lebanese negotiations said, however, that the United States apparently has concluded that there is little chance at this time for bridging the disagreements and, as a result, it is not currently playing an active role in trying to bring the two sides together, staff writer John M. Goshko reported from Washington.

[Instead, the sources in Washington said, the United States appears to be relying primarily on U.N. efforts to play a mediating role over southern Lebanon and has resisted efforts by the negotiating parties to have Richard W. Murphy, assistant secretary of state for Middle East affairs, go again to the region to explore the possibilities for U.S. mediation.]

Israel had hoped that the Naqura negotiations would lead to an agreement for an Israeli troop withdrawal accompanied by new security arrangements in southern Lebanon to protect Israel's northern border. Frustrated by the lack of progress in the talks, Israel has threatened to take "unilateral steps," risking chaos in areas evacuated by its Army.

Israel has not said how many troops it still has in Lebanon, but foreign reports have put the number at about 20,000.

Approval in principle of the staged withdrawal plan put those threats into effect and was designed in part to increase pressure on Syria and Lebanon to agree to security guarantees. Peres said the purpose of a staged withdrawal, with a pause of several months between each stage, was not only to assess the effect of each pullback but to "enable the other countries to rethink their own positions and see if we can reach an acceptable arrangement in Lebanon."

Tonight's vote, after two days of debate in the Cabinet, was the most important Israeli decision involving Lebanon since the 1982 invasion. However, the commitment to withdraw from Lebanon was less than complete, and the tentative timetable could be set back as the Israelis pull back in stages and test the reactions of local Lebanese militias, Palestinian guerrillas and the Syrians.

Peres initially hoped to win Cabinet backing for the full withdrawal plan, leaving its implementation in the hands of Rabin and the Army. But in order to shore up support for the plan, he agreed to the compromise by which the second and third stages of the pullback will also be submitted to the Cabinet for its approval.

Opposition was led by Foreign Minister Yitzhak Shamir and former defense minister Moshe Arens, now a minister without portfolio, who was quoted as terming a withdrawal without security guarantees a "dangerous gamble" for Israel's northern communities.

Another former Likud defense minister, Ariel Sharon, the architect of the invasion and a fierce opponent of the withdrawal plan, was in New York today as his libel trial against Time magazine neared its conclusion. In addition to benefiting from Sharon's absence, Peres won the backing of a key Likud official, deputy prime minister David Levy, who, along with Sharon, plans to challenge Shamir for the leadership of Likud.

There has long been strong sentiment in Israel for a troop pullback along the Lebanese coast, and particularly from the densely populated Sidon area. The sentiment was reinforced by the deaths earlier today of two Israeli soldiers and the wounding of seven by two roadside bombs in the area to be evacuated in the first stage of the withdrawal. So far, 606 Israeli soldiers have been killed in Lebanon since the invasion.

Answering questions from foreign correspondents, Peres said the staged withdrawal plan represented "an optimum of our two main goals -- namely to bring our boys back home and to ensure the security of the northern part of Israel."

Peres said Israel will take "preventive measures" if Palestine Liberation Organization forces attempt to reestablish bases in the areas evacuated by the Israeli Army.

Tonight, Rabin said that during the next five weeks Israel will suggest to Lebanon and U.N. officials that there is still time to "to organize the area to be evacuated to prevent disorders and massacres."

Israel's decision came a day before U.N. Undersecretary General Brian Urquhart is to arrive reportedly with a new plan for a redeployment of U.N. troops in southern Lebanon as part of an agreement on security guarantees. The Naqura talks have been stalled on the issue of the U.N. role and Israeli officials appear to have virtually no confidence Urquhart's proposals will break the impasse.

Special correspondent Nora Boustany reported from Beirut:

Lebanon plans to ask Urquhart for U.N. help in removing an Israeli roadblock set up just north of the Awwali River that is preventing deployment of the Lebanese Army southward to Israeli lines. About 1,700 Lebanese troops were deployed along the coastal highway south of Beirut over the weekend as a buffer force between warring Lebanese factions.

Although the coastal deployment has gone largely according to plan, Lebanese officials say they fear serious problems could arise to the south if the Israeli Army pulls out, especially between Lebanese Shiite Moslems and Christians.