Defense Minister Ariel Sharon visited Beirut last night in a last minute attempt to assure that Israel's negotiations with Lebanon will begin next week as scheduled, according to Israeli radio and the independent newspaper Maariv.

There was no government confirmation in either Israel or Lebanon of the reported trip.

Maariv reported that during the trip Sharon received assurances that Lebanon will stand by what the Israelis have described as a three-page document of "understanding" that is to serve as a guide for direct talks between the two countries.

According to other Israeli reports, in recent days Sharon has had "urgent contacts" with unidentified Lebanese who initially agreed to the understanding.

The reports said that among the issues that were not settled in the written understanding and could pose a problem in starting the negotiations are the questions of maintaining a U.N. force in southern Lebanon, which Israel opposes, and the status of the Israeli-supported Christian militia in southern Lebanon commanded by Saad Haddad, a former Lebanese Army major.

It remained unclear today whether Sharon's reported trip was prompted by serious snags in getting the negotiations started or dealt only with last-minute details -- or was an effort to enhance his political standing.

There have been a variety of denials from Beirut about any agreement, but the Lebanese also have said they are eager to begin the talks with Israel.

The Israeli government announced yesterday that direct talks with Lebanon would begin early next week.

Israel has not yet given a final reply on when it wants to start talks with Lebanese officials on withdrawing its troops from Lebanon, Reuter reported sources at Lebanon's presidential palace as saying Friday.

An Israeli government spokesman said Thursday the talks would start at the beginning of next week but did not mention a specific day.

Lebanon had been hoping the negotiations would get under way this week.

After the Israeli announcement, Lebanese Foreign Minister Elie Salem asked U.S. special Middle East envoy Morris Draper to find out exactly when Israel wanted to start.

The sources said Draper, who is currently in Beirut, was in contact with the Israelis on the issue. Leb-anon is ready but Israel has given no final reply, they added.

According to the Israeli government announcement, the negotiations are to be held on a rotating basis in Khaldah, a suburb of Beirut, and in Qiryat Shemona, a northern Israeli town.

Sharon, who has made numerous trips to Beirut since the Israeli invasion of Lebanon on June 6, announced last week that secret negotiations he had conducted with the Lebanese had produced a "breakthrough" in the negotiating stalemate in the form of a three-page understanding between the two sides. However, the document has never been released and there is considerable skepticism within Israel whether it has the formal approval of the central government.

Sharon has taken credit for the breakthrough and has the largest personal stake in seeing that the negotiations actually take place.

He has been criticized widely for waging a costly war without gaining political advantages.

The document, according to Israeli officials, sets out general principles covering an end to the formal state of war between Israel and Lebanon and the beginning of normal relations between them, the establishment of a "security zone" for Israel in southern Lebanon that would give the Jewish state, among other things, overflight rights, and an Israeli troop withdrawal from its northern neighbor's territory.

The negotiations in Khaldah and Qiryat Shemona are to work out the details of these general principles, according to Israel.

However, any agreement between Israel and Lebanon also depends on the willingness of the Syrian and Palestinian forces to withdraw from Lebanon.

The Israelis are demanding that the estimated 7,000 Palestinian guerrillas leave the country before it withdraws its troops simultaneously with the Syrians.