UNITED NATIONS, AUG. 28 -- The five permanent members of the Security Council announced agreement today on a peace plan to end the 11-year war in Cambodia that could pave the way for one of the largest and costliest operations in U.N. history.

The plan must be accepted by the four warring factions in the conflict -- the Vietnamese-backed government in Phnom Penh and three guerrilla groups joined in a coalition -- which are scheduled to meet in Jakarta, Indonesia, next month.

Thus far, the two non-Communist groups in the guerrilla coalition have endorsed the agreement. There has been no response from the Communist Khmer Rouge guerrillas or the Vietnamese-backed Cambodian government of Prime Minister Hun Sen.

In the past, the factions have appeared to reach agreement on issues only to resume discord. But diplomats here said the situation is different now because the big powers -- China, the United States, Britain, France and the Soviet Union -- are united on a framework for peace for the first time after years of supporting different sides in the conflict. The diplomats expressed hope that this unanimity would have a positive impact on the Cambodians.

U.S. and Soviet officials said that if the Cambodians accept the plan, a cease-fire could be in place by the end of this year. "If everything goes smoothly, maybe it will take a month or two for a cease-fire," Soviet Deputy Foreign Minister Igor Rogachev told reporters after the agreement was announced. "I am sure all the countries concerned finally will cease their military supplies, and this moment is coming."

A U.S. official, speaking on the basis of confidentiality, predicted that U.N. peace-keeping forces could begin taking up positions in Cambodia "by the end of the year if the four factions in the dispute agree."

Should the Cambodians agree to the five-point peace plan, it would mark the beginning of a U.N. operation unprecedented in cost and use of civilian personnel.

The U.S. official said estimates call for the involvement of up to 10,000 military personnel and 10,000 civilians. This could cost $3 billion to $5 billion, depending on the length of U.N. involvement, which he said could run one to two years.

Logistical problems associated with the operation are said to be enormous. The U.S. official described the water in war-ravaged Cambodia as being "practically dangerous," and transport and telecommunication links are said to be almost negligible.

The five-part peace plan deals with the administration of Cambodia during a transitional period, military arrangements, elections, human-rights protection and international guarantees relating to Cambodian sovereignty.

The agreement calls for the parties to the Cambodian conflict to form a supreme national council until free elections are held. While the council would nominally govern the country, diplomats familiar with the plan said, the United Nations would likely take control of up to five government ministries -- defense, foreign affairs, finance, public security and information.

Still unresolved is the composition of the supreme national council. The Khmer Rouge has been calling for all factions to have equal numbers of seats. But a Western diplomat said it could involve a formula that gives Hun Sen's government six seats and the three other factions two each.

The United Nations would also have a large role in a three-part process of disarming the various military factions.

The agreement represents a significant shift by China, which supplies and arms the Khmer Rouge guerrillas. The Khmer Rouge was responsible for the deaths of up to 2 million Cambodians when it ruled in Phnom Penh from 1975 until being ousted by Vietnamese troops in January 1979. Fears that it might take power again fueled diplomatic efforts to end the conflict.

In a statement, the five powers called on the four factions "to commit themselves" and to "exercise maximum self-restraint so as to create the peaceful climate required to facilitate the achievement and the implementation of a comprehensive political settlement."

The main obstacle, the U.S. official said, is the deep hostility among the Cambodian factions. But, he said, "We think there are grounds for confidence that the political momentum will lead to consensus."