The five permanent members of the U.N. Security Council are expected to announce agreement today on a proposed peace plan to end the 11-year civil war in Cambodia in which the United Nations would assume a major role, congressional and administration sources said.

A pact would mark the first time that the five countries -- the United States, the Soviet Union, China, Britain and France -- have reached agreement on all aspects of ending the war, despite having represented competing sides in the conflict. In particular, an agreement would mark a change in policy for China -- the major holdout to such an approach -- which has been trying to improve its international reputation since crushing pro-democracy demonstrations last year.

However, the sources said the plan must be accepted by Cambodia's four warring factions -- the Vietnamese-backed Phnom Penh government and the three-party guerrilla coalition that includes the Chinese-backed Communist Khmer Rouge -- and it is not clear that all will approve it.

The four factions are scheduled to meet next week in Jakarta, where they will be asked to form a Supreme National Council to represent Cambodia during an interim period leading to free and fair elections. The composition of such a council, however, is still in dispute by the Cambodian factions.

A peace plan that includes a major role for the United Nations would be in keeping with the approach the United States and the Soviet Union have been advancing toward resolving regional conflicts -- including Afghanistan and Namibia.

U.S. officials are expected to meet with officials from Vietnam this week in an effort to persuade the government in Phnom Penh to accept the new peace formula, a source said. The United States began a dialogue with Vietnam on the subject of Cambodia after dropping its diplomatic recognition of the guerrilla coalition in July because of fears the Khmer Rouge might fight its way back into power.

A State Department official said he was "optimistic" a pact would be agreed upon this week but said work was proceeding yesterday on details. The official said the proposal is expected to meet the U.S. policy goals of verifying withdrawal of Vietnamese troops from Cambodia, preventing the Khmer Rouge from taking power and providing for self-determination for Cambodians.

Specifically, the official said, the agreement is expected to include steps to control the warring military forces, arrangements for setting up an interim authority and plans for elections under U.N. auspices.

Ambassadors and deputy ministers from the permanent members of the Security Council, which have been negotiating on the issue for six months, met yesterday to put final touches on the proposals, which were to be presented to Secretary General Javier Perez de Cuellar.

Solutions had been reached on the key issues of disarming the factions and setting up a workable structure in Phnom Penh for an interim government prior to free elections, the sources said.

Under the reported plan, the United Nations would play a large role in administering Cambodia until elections could be held. Because it is assumed that the four factions will be unable to reach broad enough agreement to permit them to run an interim government, the Supreme National Council would turn over most of its powers to the world body until a new government is elected.

The Phnom Penh government of Prime Minister Hun Sen is battling the guerrilla coalition that includes the Khmer Rouge, which is responsible for the deaths of up to 2 million people when it ruled Cambodia from 1975 until being ousted by Vietnamese troops in January 1979.