A diplomatic arrangement to end the threat of military conflict between Israel and Syria in southern Lebanon is very close to agreement with the aid of U.S. mediation, informed sources said yesterday.

The arrangement, which probably will not be formally announced, will involve the phased withdrawal of Syrian troops from a sensitive area of southern Lebanon close to the Israeli border, the sources said. Israel has strongly forces into the area and hinted at potential military action to counter it.

The U.S. mediation involving meetings here and in the Middle East is likely to bring a settlement of the problem, at least in principle, before Secretary of State Cyrus R. Vance leaves Monday on his Middle East factfinding trip, according to these accounts.

The reports of impending success in the U.S. diplomatic efforts involving Southern Lebanon came as bipartisan Senate group issued an optimistic statement about the chances of a comprehensive settlement of the overall Arab-Israel dispute.

The congressional statement, from a study group headed by Sen. Abraham A. Ribicoff (D-Conn.) and Senate Minority Leader Howard H. Baker Jr. (R-Tenn), put renewed emphasis on the U.S. peacemaking role which Vance is to explore next week in a flying trip to Israel, Egypt, Lebanon, Jordan, Saudi Arabia and Syria.

The senators, reporting that Arab leaders are now willing to recognize the right of Israel to exist, called on the United States to take a leading part in comprehensive Middle East peace negotiations expected later this year. The senatorial report arose from a mission by the lawmakers to the region last November.

The conclusions of the 12-member senatorial group were based on lengthy discussions with President Anwar Sadat of Egypt and King Hussein of Jordan as well as Israeli Prime Minister Yitzak Rabin and other leaders.

The lawmakers did not see Syrian President Hafez Assad and Yasser Arafat. The militant chairman of the Palestine Liberation Organization, who are also reported to modifying their position on possible deals with Israel.

According to the senators, two basic points represent significant changes in the Arab position:

"For the first time Arab leaders are willing to recognize the right of Israel to exist as an independent and secure Jewish state." Such recognition and an eventual settlement would probably involve some kind of international guarantee, they said. The senators quoted Sadat as saying he has "no objection" to giving Israel whatever security guarantees it wants from the United States, the United Nations or other sources, assuming Egypt receives the same guarantees.

Sadat and his foreign minister, Ismail Fahmy, both said that Egypt would start peace negotiations without preconditions." The senators said this stand "was an important departure from previous Arab postions and presents an opportunity to move forward."

"Both Sadat and Fahmy said that no previous assurances were required for negotiations to restore territories, to end belligerency, in respect of the PLO, the status of Jerusalem or for the future of conventional arms and nuclear weapons in the area but all of these issues would have to be negotiated thoroughly" at a new Geneva peace conference, the senators said. They added that, in effect, the Egyptians had agreed to talk and set forth an agenda but did not insist on these conditions.

"There appeared to be a willingness to blur the position of the PLO: rather than accord it a status as a government in exile, the Arabs were prepared to consider it part of an overall 'umbrella' delegation or negotiating group," the senators said.

The lawmakers found less optimism in Israel about the possibilities of an overall Middle East settlement, and doubts about the Arab position. "Israeli officials repeat in different ways that Arab nations are, not prepared to recognize Israel's right to exist as a nation," the senators said.

On substantive issues that would be important to any settlement, "Rabin said that not all territories [occupied by Israel in 1967] would be returned; Jerusalem would never revert to Arab countrol," the senators said. Nevertheless, Foreign Minister Yigal Allon was quoted as expressing confidence that if negotiations could be started, progress could be made.

"All sides agreed that no serious progress would take place without an American lead," the senators reported.

The situation regarding southern Lebanon has been tense since Syrian troops entered the Nabatiyeh are, only seven miles from Israel's border, on Jan. 23. The arrival of the Syrian force, estimated at no more than 500 men, drew quick Israeli demands for their withdrawal, made public in Israel and transmitted diplomatically to Syria and Lebanon by the United States.

The Syrian troops are part of a 30,000-strong Arab League force that is policing the truce in Lebanon. The Syrians have maintained that the movement of its troops was necessary to bring southern Lebanon under assemblance of order.