Apparently fearful that Israel has launched its final drive to destroy the western sector of Beirut, French President Francois Mitterrand issued an urgent appeal to the United Nations yesterday to intervene in Lebanon to separate the attacking Israeli forces and the Palestinian and Lebanese leftist units they have trapped.

French officials in Paris acknowledged that Mitterrand's appeal, made in the form of a request for a Security Council meeting in New York to consider a U.N. military observer force, had little chance of success. Foreign Minister Claude Cheysson was quoted by Agence France-Presse as saying that the United States would probably veto such a move because the Reagan administration "is totally in the hands of the Israelis."

The decision by Mitterrand to try at the last moment to mobilize international opinion against the continuing Israeli assault brought the French Socialist leader to the point of an open break with Israel and the United States. Mitterrand had worked hard to make friends with these two countries after years of estrangement under previous French governments.

Consultations within the 15-member council will begin today. French diplomats at the United Nations insisted last night that France did not want a confrontation with the Reagan administration. They initially sought closed-door consultations among council members, which might have produced a compromise package, but that fell through when other council members declined to move into those discussions immediately.

The statement issued at the Elysee Palace said: "There is every reason to fear that in the coming hours tragic combat in Beirut will add to the suffering already endured by the people of Lebanon," a former French mandate. "The destruction of the Lebanese capital would lastingly compromise the future of the entire country and peace in the entire region."

Israel, which welcomed Mitterrand in March as the first French president to visit the Jewish state, responded sharply to the French initiative. Israeli Foreign Minister Yitzhak Shamir was quoted by a ministry spokesman in Jerusalem as ruling out any French role in an international force in Lebanon, and Cheysson disclosed that Shamir had criticized him in a telephone call for saying that Israeli policy toward the Palestinians was "suicidal."

Cheysson, an outspoken diplomat whose blunt assessments have made him the subject of considerable controversy in France during the Socialists' first year in power, told journalists in an informal conversation after Mitterrand's statement was issued that Paris regretted that the Reagan administration was not opposing Israel's assault.

The rift over the Middle East came as France, which had provided important support for the Reagan administration's strategic rearmament and defense programs, and West Germany are expressing deep anger over the Reagan administration's pressure on them to cut back on trade with the Soviet Union and to give up a planned pipeline that would bring natural gas from the Soviet Union to Western Europe.

The French proposal on Lebanon was not coordinated with the United States, which has been seeking an accommodation in Lebanon through special envoy Philip C. Habib, and also caught France's European Community partners by surprise. American officials have been reluctant to involve the United Nations, and were unenthusiastic about a successful French resolution last weekend to provide U.N. relief supplies and humanitarian help to Lebanon.

U.S. officials said that consultations would begin in the Security Council today after the French proposal has been referred "to the highest level of our government." French Ambassador to Washington Bernard Vernier-Palliez met with Undersecretary of State Lawrence J. Eagleburger in Washington yesterday.

The Lebanese government is expected to respond Friday to the statement, which says the U.N. force suggested by Mitterrand would have to be invited by Lebanon and would have to work with the Lebanese Army to separate the warring armies.

Mitterrand's office issued the communique saying he wanted West Beirut put under the control of U.N. observers and "neutralized" by the disengagement. The statement singled out Israel for an appeal to observe the cease-fire that had been declared before the massive Israeli bombardments of the past two days.