Senior administration officials said publicly for the first time yesterday that their "target date" for a withdrawal of Israeli, Syrian and Palestine Liberation Organization forces from Lebanon is "the end of the year."

The officials noted that Israeli Foreign Minister Yitzhak Shamir recently said publicly that a withdrawal of all forces from the war-torn country could be carried out by the end of the year. But yesterday marked the first time that any top officials here had put a specific timetable on the process and agreed with the Israeli assessment.

"We think that's a great time," the official added about the end-of-the-year estimate, noting President Reagan's expressed desire to get all the forces out as fast as possible. He said Nov. 22, which is Lebanon's National Day, "would be better, but that's a bit unrealistic."

The withdrawal of the three warring forces from Lebanon would be a first step toward what the Reagan administration hopes will be eventual stability for the new Lebanese government of President Amin Gemayel, to be followed by withdrawal of U.S. Marines now in Lebanon as part of a multinational peace-keeping force, and a broader peace between Israel and its Arab neighbors.

The officials who briefed reporters yesterday on Gemayel's visit to this country -- he arrived in Washington last night -- did so under rules which do not allow them to be named.

The officials said that arranging the withdrawal will be "complicated and difficult" but expressed optimism that it can be done. "The issue is what kind of preconditions the various parties will try to hang on" the withdrawal.

Aside from basic problems of arranging the terms and timing for withdrawal of more than 30,000 Syrian troops from eastern and northern Lebanon and some 8,000 remaining PLO fighters, a key issue is how to satisfy Israel's demand for a "security zone" 25 to 40 miles deep in southern Lebanon just above the Israeli border.

After Shamir's talks here last week, Israeli officials in Jerusalem said agreement on establishing such a zone could come soon. American officials have said the talks with Shamir went well.

But a key issue is said to be an Israeli desire that the Israeli-equipped and -supported Christian Lebanese militia forces led by former Lebanese Army Maj. Saad Haddad play a role in the security zone. Some American officials feel this could be as hard to arrange as the Syrian or Palestinian pullouts.

With the Lebanese Army weak and untested, Israelis believe Haddad's forces, as they now exist or eventually as part of an expanded army, are crucial to their security interests and that Haddad must somehow remain their leader. But if Haddad's forces are not brought under effective control by Gemayel's government, it could weaken that government's authority. The role of Haddad's forces will be discussed with Gemayel today, officials said.

The Israelis, as other officials here explain it, want the security zone agreement with Lebanon not only to keep the PLO fighters from infiltrating back into Lebanon and close to the Israeli border, but also because it is tantamount to recognition of Israel by Lebanon.

The Israelis have said they ultimately want a peace treaty with Lebanon. It is widely agreed that such an agreement is politically difficult now because it could cause trouble for Gemayel's new government elsewhwere in the Arab world. So the security zone agreement is seen as the first step in an Israeli strategy which, as in the Camp David agreements that produced peace with Egypt, is aimed at drawing Arab countries into peace accords one by one.

Officials briefing reporters yesterday declined to comment on a variety of speculative proposals that are being informally discussed about how that southern security zone might eventually be patrolled and by whom.

They also declined to discuss whether the Reagan administration, after lengthy discussions abroad and the meetings with Shamir, was ready to put forward any specific points upon which they felt agreement could be reached as steps toward a withdrawal.

Officials said the discussions with Gemayel would have to be completed first and then further discussions with the Syrians and then, through intermediaries, with the PLO. They left open the possibility that there could be another meeting with Shamir later this week when the Israeli official, now in Costa Rica, passes through New York en route back to Israel.

The 40-year-old Gemayel will meet with Reagan, Secretary of State George P. Shultz, Secretary of Defense Caspar W. Weinberger and World Bank officials here today.

The briefing officials said it was not likely that the United States would announce any new economic aid for Lebanon during the visit. Rather they said the significance of the visit goes well beyond the acknowledged need for more aid.

The meetings here will be, in effect, the international debut of the young president, an official said, "who has just taken over in circumstances of great tragedy and who is determined to make a reality out of a vision of the future of his country. It is going to be an extremely important visit politically and psychologically."

Pentagon officials, awaiting the return of a survey team from Lebanon, estimate that it is now going to take more money and equipment than originally thought to expand the Lebanese Army from its current 20,000 troops to perhaps 40,000 and to improve training.

Anerican officials estimate that it will probably cost $10 billion to $15 billion to rebuild Lebanon but made clear the United States was not talking about huge aid coming from this country. They said the United States had already provided some $105 million in increased economic help, that there was a large private effort under way in Lebanon, and that "we are not going to do it for them. We are going to assist" and encourage others, especially the World Bank, to take the lead.