Despite an apparent setback to American policy caused by the killing of Lebanon's President-elect Bashir Gemayel, the Reagan administration is pressing forward with its recently announced Middle East peace initiative.
Publicly, the administration is admitting that Mr. Gemayel's assassination has ''complicated'' its quest for solutions to the Lebanon turmoil and the Palestinian issue. Privately, some analysts express concern that:
* Attention in the Middle East has been at least temporarily diverted from President Reagan's plan.
* A withdrawal of Israeli and Syrian forces from Lebanon has been delayed. This, analysts say, heightens the chance of further Israeli-Syrian clashes or an Israeli-Syrian partition of Lebanon.
* The chances of forming a strong central Lebanese government - a necessity for progress on other fronts in Lebanon, in Washington's view - have suffered a considerable setback.
Washington's ability to influence events appeared to some observers to be limited at this juncture, and Israel appeared once again to be seizing the initiative. On Sept. 15, the day after Bashir Gemayel's assassination by a huge bomb, which also claimed the lives of at least 20 others, Israeli armored forces entered West Beirut. Israel's ambassador in Washington, Moshe Arens, argued that Israel was now the only force for stability in the country. President-elect Gemayel, the tough commander of a militia group, had been heavily supported by Israel. But he had also taken steps to broaden his base of support and to reach out to the country's Muslim community.
Lebanese officials objected to the Israeli argument that the Israelis' move into West Beirut helped to stabilize the situation. Lebanon's ambassador to the United Nations, Ghasan Tueni, told the Monitor that some observers, including Israelis, were overstating the danger that factional warfare would once again erupt in Lebanon unless the Israelis maintained control.
''The fear of factional fighting has been overtaken by events,'' said Mr. Tueni. ''If it was going to happen, it would have happened by now. . . . The reaction of the Muslims to all this has been superb.'' The ambassador said that Lebanon's House of Parliament was expected to meet soon and to remain in session until it had elected a new president. The chances that a new president would be elected unanimously were good, the ambassador said. One possibility is that the parliament will ask President Elias Sarkis to extend his term.
State Department officials said Israel told the US its move into west Beirut was ''very limited and precautionary.''
One official said, however, that the Lebanese armed forces (LAF) had been ''doing well'' in taking control of the situation in Beirut and that such Israeli moves could weaken the Lebanese government rather than strengthen it.
''If it becomes apparent that every time the LAF faces difficulty, the Israelis are going to send their tanks in, that is not going to strengthen the authority of the central government,'' the official said. If the Israeli move into west Beirut goes beyond being ''very limited,'' it could open a new rift between Israel and the US.
''The usual optimistic American idea of a strong central government in Lebanon is not the idea which the Israelis are talking about,'' said L. Dean Brown, a special US envoy to Lebanon under the Ford administration. ''We are talking about an independent government, but the Israelis are talking about one which will be dependent on them.''
''The chances of getting a strong central government are now slimmer,'' said Mr. Brown, who is director of the Middle East Institute here. ''I see a far greater chance that the Israeli, Syrian, and remaining PLO forces will now have at each other again.''
State Department officials, meanwhile, were highly encouraged by an interview that King Hussein of Jordan gave to the British Broadcasting Corporation on Sept. 13. In the interview, Hussein said he believed some form of a plan for a federation of Palestinians and Jordanians would emerge and that Arab recognition of Israel will be the inevitable outcome of peace talks in the region. But Hussein also reaffirmed the key role of the Palestine Liberation Organization in any Middle East settlement and stated that there could be no question of any compromise over Arab claims to East Jerusalem. Hussein called Reagan's peace plan, announced earlier this month, ''the most courageous stand taken by an American administration ever since 1956.''
''This is very encouraging,'' said a State Department official. ''Hussein is saying all the right things.''