The United States has accepted Israel's decision for a partial pullback of its forces in Lebanon and will not press the Jerusalem government to delay the redeployment, diplomatic sources said last night.

President Reagan, evidently reflecting this decision, said last night that he hoped Israel would see its planned redeployment of troops in Lebanon as "one phase" of a withdrawal of all foreign forces from that country and that it would encourage the Syrians to take part in the process.

But he expressed U.S. concern, shared by the Lebanese, that a partial Israeli withdrawal might turn out to mean a de facto partition of the country.

"I think," Reagan said in his nationally televised news conference, "there is fear if there is simply a withdrawal to another line and a digging in, and fortifying along that line, that this would be what it looks like Syria is doing, and that is simply trying to partition Lebanon, reduce Lebanon and grab off some territory for themselves."

The president's remarks came after a day of talks between the visiting Israeli defense and foreign ministers and senior administration officials during which the redeployment question, the situation in Lebanon and the ability of the Lebanese Army to replace the Israelis were discussed.

Foreign Minister Yitzhak Shamir and Defense Minister Moshe Arens and their delegation spent 5 1/2 hours in talks with Secretary of State George P. Shultz and Reagan's newly appointed Middle East envoy, Robert C. McFarlane. Nicholas A. Veliotes, assistant secretary of state for Near Eastern affairs, and Samuel Lewis, ambassador to Israel, also took part.

The presence of Israel's chief of military intelligence, Gen. Ehud Barak, was seen as an indication of the detailed nature of the discussions.

Shultz also updated the Israelis on last week's visit here by Lebanese President Amin Gemayel, who reportedly expressed concern about the Israeli plans for a partial pullback, a concern shared by some administration officials.

Diplomatic sources said yesterday, however, that they believe the United States will seek detailed coordination with the Israelis so that their timetable can match U.S.-backed efforts to bolster the effectiveness of the Lebanese government and army.

"There was no pressure on Israeli deployment. There won't be that kind of pressure. The Americans did not ask the Israelis to delay," Arens said after yesterday's discussions.

The president expressed the hope last night that a partial withdrawal "will be recognized and admitted to be, by the Israelis, one phase of their agreement to withdraw . . . . It certainly will give us a better case for breaking the roadblock that has been established by Syria and persuading them the Syrians to keep their original promise that when others withdrew they would withdraw."