The United States and the Soviet Union launched a coordinated diplomatic effort in Washington yesterday to persuade both sides in the 15-year-old Angolan civil war to reach agreement on a cease-fire by early next year.

Soviet Foreign Minister Eduard Shevardnadze, whose government has poured billions of dollars in aid and arms into Angola to prop up the leftist government, met for the first time with the U.S.-backed Angolan rebel leader Jonas Savimbi.

At the same time, Secretary of State James A. Baker III held talks with Angolan Foreign Minister Pedro de Castro Van Dunem, who arrived in Washington yesterday morning for the unusual set of meetings.

The principal goal of both 45-minute discussions, diplomatic sources said, was for each faction to hear directly from the United States and the Soviet Union that the international political situation is, as Shevardnadze told Savimbi, "quite propitious" for resolving the conflict and that the superpowers are united in that view.

While cautioning that many critical issues remain to be resolved, diplomatic sources said the intervention by the superpowers was essential to help end the fighting that has killed many thousands of people.

Although the two warring sides in Angola have been engaged since April in peace talks brokered by Portugal, they need "the political and military guarantees that only the superpowers can give," said Antonio Monteiro, a senior Portuguese Foreign Ministry official, who also is meeting here with the factions and with Soviet and U.S. officials.

The Shevardnadze-Savimbi meeting, a State Department official said, was "a very significant breakthrough," giving Savimbi some of the political recognition he has sought but has been denied by the Luanda government. After the meeting, Savimbi said the two had discussed the "problems and the possibilities of implementing a cease-fire."

Savimbi recently dropped his demand for formal recognition, and Angola's ruling party on Monday approved the establishment of a multi-party system, discarding its Marxist-Leninist ideology. "The question now is whether the {Angolan government} is prepared to put its fine words into practice" and "test themselves in the political arena," the State Department official said.

The meetings, which are to resume today, followed agreement on Tuesday in Houston between Baker and Shevardnadze to seek to improve chances that peace talks scheduled next month in Lisbon will produce arrangements for a cease-fire and elections. The United States and the Soviet Union have agreed that, at some point after a truce takes effect, they would cut off arms to their clients.

The intensive Soviet-U.S. cooperation on Angola is an extraordinary turnabout on an issue that was once a key source of tension between the two countries.

The leftist government in Angola came to power in 1975 with the help of the Soviet Union and 50,000 Cuban troops. The last of the Cubans are expected to leave by July, and the Soviets have said they intend to cut military assistance to Angola.

Congress last October agreed to hold back half of the estimated $40 million in lethal aid to Savimbi. It said the remainder could be withheld if the Angolan government accepted a cease-fire and proposed a "reasonable timetable" for free and fair elections and if the Soviets stopped giving lethal aid to the Angolan regime and withdrew its advisers.

Shevardnadze, after his meeting with Savimbi, said yesterday that "if there is an agreement on a cease-fire, then it would be easier for us to address the issue of a cutoff of arms."