The United States and Lebanon reacted warily yesterday to the partial troop pullback announced by Israel as Lebanese President Amin Gemayel began high-level talks in Washington.

U.S. statements at the White House and the State Department neither approved nor criticized the Israeli cabinet decision. The statements took the position that any partial pullback should be "within the framework" of a hoped-for complete withdrawal and "should not complicate the very difficult tasks facing the government of President Gemayel," but officials would not say how the projected Israeli action would fit these guidelines.

In the past, U.S. officials have insisted that a partial pullback by Israel be linked to a full withdrawal of forces. The reference to the impact on Lebanese governmental tasks is new, evidently reflecting U.S. concern about how Israel's action is being seen by the fledgling Beirut regime.

Official Lebanese sources quoted Gemayel as telling members of Congress yesterday of his fear that a partial withdrawal of Israeli forces could lead to a de facto partition of Lebanon unless the troop movements take place in the context of a plan for a total withdrawal of all troops. At this stage, the sources said, Lebanon is not in a position to make a definite judgment on the Israeli plan, having heard no details and nothing official from Jerusalem.

Gemayel's impromptu remark to reporters in a Capitol Hill hallway was less cautious. "We are against the partial withdrawal . . . .It means a de facto partition," United Press International quoted the Lebanese president as saying after a closed-door meeting with lawmakers.

Lebanese Foreign Minister Elie Salem also spoke with implied criticism of the Israeli decision. "After eight years of war and destruction, Lebanon is in no mood to talk about an army withdrawal of one or two kilometers because we are talking about the life of the nation and the survival of democracy," Salem told reporters. He added, "We are not interested in merely accommodating the external political interests of other countries."

There was much speculation in both U.S. and Lebanese official circles about the timing of the Israeli cabinet action, which came on the heels of Prime Minister Menachem Begin's sudden postponement of his planned trip to Washington next week, and just before Gemayel's scheduled discussions here today with Secretary of State George P. Shultz and with President Reagan on Friday.

A senior State Department official briefing reporters at the White House said the Israeli announcement "insures that the subject of the partial pullback will be discussed" in the forthcoming talks with Gemayel. For Israel to have announced the partial withdrawal after the U.S.-Lebanese talks were finished could have produced a more awkward situation, according to another State Department official.

Another Mideast event yesterday, the daylight shelling of Christian neighborhoods in East Beirut from Syrian-controlled mountains, was ascribed by Lebanese officials here to international maneuvering over the future of Lebanon.

A member of Gemayel's delegation, charging that Syria is "in a mood to escalate politically and militarily," said the shelling appeared to be intended to embarrass the Lebanese government during the Washington meetings "to show it doesn't pay to follow U.S. initiatives."

State Department spokesman John Hughes called the shelling "brutal and indiscriminate" and said it is "especially distressing at the very moment the Lebanese president is engaged in talks here" on troop withdrawal and other questions.

Syria's refusal to consider withdrawal of its troops from Lebanon in parallel to a negotiated Israeli pullout was described in a White House news briefing yesterday as "the key problem" in the current phase of diplomacy regarding Lebanon.

"We'll have to work around it," said the briefer, a senior State Department official dealing with the region who cannot be identified under the terms of the briefing.

The briefer said the Lebanese demand for new momentum toward full withdrawal of foreign forces has created "an urgent environment" for this week's Washington talks. But he gave no hint of what steps are being considered to restart the momentum.