Israel announced today that it will begin direct negotiations early next week with Lebanon on troop withdrawal, but the talks are being viewed here with considerable skepticism despite the Israeli government's public optimism.

Prime Minister Menachem Begin's office announced the start of the long-awaited negotiations and said the first session would probably be held in the Beirut suburb of Khaldah. The exact date has not been set, but it is expected to be Monday or Tuesday, with the talks alternating between Khaldah and Qiryat Shemona in northern Israel.

Despite a variety of denials from Beirut, Israeli officials today defended an unsigned three-page document approved by the Israeli Cabinet Sunday as a joint Lebanese-Israeli agreement setting out the principles to be discussed in the formal negotiations.

Israel has not named the Lebanese officials with whom Defense Minister Ariel Sharon and David Kimche, director-general of the Israeli Foreign Ministry, reportedly negotiated the agreement, and the Lebanese government of President Amin Gemayel has denied that it has given its approval.

But an Israeli official said today that the document, written in English, "has backing from Lebanese authorities," and that it "is the result of an understanding reached with persons who had the authority to reach such an understanding."

"The talks will be held to transfer into practice the principles that are mentioned in the paper," the official said.

In addition to questions about Lebanon's endorsement of the document, there are also differing views on its purpose and on what the U.S. role should be in the Israeli-Lebanese negotiations.

Arab sources in Lebanon and in Washington have suggested that, rather than negotiating with the Lebanese government, Sharon dealt only with Phalangist officials of the Israeli-supported Christian militia in Lebanon, which is close to Gemayel.

The U.S. State Department is understood to be fully satisfied, in any case, that Sharon negotiated with someone authorized to speak for the Lebanese government.

Israeli officials have noted that before Sunday's Cabinet meeting that approved the document, Begin met privately with U.S. special envoy Philip C. Habib, who had just returned from Beirut and a meeting with Gemayel. The implication here, although never stated directly, is that Habib conveyed to Begin Gemayel's personal assurances that Lebanon would live up to the content of the document, which has not been made public.

U.S. sources have said only that Habib conveyed to Begin his own personal satisfaction that the document contained issues that Lebanon agreed to negotiate.

Israel and Lebanon, in the view of U.S. officials, see the document differently, with Israel presenting it as much more of a formal agreement. Lebanon, these officials say, considers it nothing more than a list of issues that it agrees would be reasonable to discuss.

Habib's colleague, U.S. envoy Morris Draper, is expected to take part in the Lebanese-Israeli talks but even before they have begun, there is disagreement over the U.S. role.

The Lebanese have spoken in terms of "tripartite negotiations," suggesting that the United States would play a role similar to the one it played in the Egyptian-Israeli peace negotiations. But Israeli officials insist that the U.S. delegation will include observers only, not "full partners" in the talks.

U.S. officials, while trying to avoid the comparison with the Egyptian-Israeli talks, say they expect the American role to be virtually that of "full partner."

A prime aim of Israel has been direct negotiations with Lebanon, without the participation of other parties or the appearance that only military matters were under discussion. It sees such direct talks as a tacit recognition of Israel's legitimacy as a state by a second Arab country after Egypt.

Lebanon is primarily interested in gaining an Israeli troop withdrawal and has drafted its own working paper putting that item at the top of the negotiating agenda.

Attempts to begin the negotiations were stalled for weeks until Sunday, when the Israeli Cabinet, in addition to approving the preliminary document, dropped its insistence that Jerusalem be one of the sites for the talks.

According to Israel, the understanding outlines three main topics that are to be settled: an end to the formal state of war that has existed between Israel and Lebanon since Israel's creation in 1948 and establishment of "normal relations," with an open border and the free flow of trade and tourism; the establishment of "security arrangements" for Israel in southern Lebanon, including early warning stations and Israeli overflight rights; and then the withdrawal of Israeli troops from Lebanon.

The success of the Israeli-Lebanese talks hinges on the success of separate negotiations to gain Syrian and Palestinian troop withdrawals from Lebanon.

Israel announced today Kimche will head its six-man delegation to the talks. Others will be Maj. Gen. Avraham Tamir, an adviser to Sharon; Elyakim Rubenstein, the Foreign Ministry's legal adviser; Yitzhak Lior, the Israeli Foreign Ministry's representative in Beirut; Brig. Gen. Amos Gilboa, an Army intelligence officer, and Brig. Gen. Uri Saguy, a member of the Army general staff. -