Israeli officials are pleased at strong backing from the Reagan administration for a plan to partially withdraw troops from Lebanon - essentially the same plan as that which the Americans feared only weeks ago would lead to de facto partition of Lebanon.
But officials here are taking pains to scotch press reports that Israel will ultimately pull back all its troops from Lebanon, even if 40,000 Syrian soldiers remain.
Israeli Cabinet Secretary Dan Meridor said Sunday that Israel still insists on simultaneous withdrawal with the Syrians and Palestine Liberation Organization troops in Lebanon. At a Cabinet meeting, where Foreign Minister Yitzhak Shamir briefed members on his talks in Washington last week, there was no discussion of setting an overall timetable for withdrawal from Lebanon.
Mr. Meridor said there had been no change in the Israeli position on the withdrawal question. However, even a partial pullback represents a shift from Israel's original position that its troops would stay put in Lebanon until Syria's simultaneous exodus.
The Israeli pullback, which could begin this week and conclude within three months, involves Israeli evacuation of about 90 square miles of territory, including the southern outskirts of Beirut and the Shouf Mountains east of Beirut. This would leave Israel on the Awali River, 27 miles north of the Israel-Lebanon border.
Israel's position in east Lebanon's Bekaa Valley facing the Syrian Army would remain the same. Syrian opposition has blocked implementation of the May 17 Israel-Lebanon accord on withdrawal of Israeli troops.
Robert McFarlane, who replaced Philip Habib as the American envoy to the Mideast and who may be better received in Damascus than Mr. Habib, is visiting Syria as part of a tour of the region to seek ways of achieving an overall withdrawal of foreign troops from Lebanon.
As late as last week, when Israeli Defense Minister Moshe Arens and Foreign Minister Shamir left for Washington, Israeli officials believed the United States would try to persuade them to halt or at least delay redeployment. The US feared an Israeli pullback might destabilize the government of Lebanese President Amin Gemayel by creating a power vacuum in areas which the Lebanese Army was not yet ready to secure from withdrawing Israeli forces.
But US officials dropped their opposition. President Reagan went so far as to say in a Washington television interview over the weekend that redeployment was ''welcome'' because it ''gives us some needed leverage to use against the Syrians.''
What changed the Americans' minds?
Some observers here believe that Israeli determination to move ahead, borne of domestic pressure to cut back Israeli casualties in Lebanon, left the Reagan administration no choice. Moreover, the Americans were arguing from an awkward position, having previously and wrongly assured Israel that the Syrians would leave once Israel and Lebanon signed an accord.
But, more important, the partial accord was made easier to swallow by describing it as a first step in implementing the overall Israel-Lebanon accord. This concept was first broached three weeks ago by Israeli officials during the visit of Secretary of State George Shultz to the Mideast.
But the US would like Israel to set a fixed timetable for total unilateral withdrawal. Such a date, some American officials believe, would eliminate any excuse by Syria to stay in Lebanon, and would increase pressure by moderate Arab states on Damascus to leave. It would also allay continuing fears by the Lebanese government that the Israeli partial pullback spells partition of their country, despite Israeli disclaimers.
The US would at least like a timetable for Israeli evacuation of south Lebanon. Sources here say the Americans differentiate between south Lebanon and east Lebanon's Bekaa Valley, where the Israeli presence, face-to-face with the Syrians and only 16 miles from Damascus, is seen as an important source of pressure on the Syrians to leave.
But Israel has so far refused to spell out whether the new formulation defining partial withdrawal as ''gradual implementation'' of the Israel-Lebanon accord, in the words of Deputy Foreign Minister Yehuda Ben Meir, means that Israel will eventually pull all the way back to its border, even if the Syrians stay in Lebanon.
Foreign Minister Shamir said on his return from Washington that there had been ''talk'' there ''about the possibility of a timetable, but we explained that we couldn't do that because we had no idea of Syria's plans or the PLO's.''
One senior Israeli minister said after Sunday's Cabinet meeting that Israel might consider setting a target date for total withdrawal of its forces, but only on condition that the Syrians withdraw along with Israeli troops.
Mr. Shamir said the US had not raised any new ideas in Washington for persuading Syria to pull out of Lebanon. He reiterated that Israel would remain there as long as Syria did. This could mean that Israeli troops will be in for a long stay unless US envoy McFarlane can persuade the Syrians to be more flexible than they have been until now.