JAKARTA, INDONESIA, SEPT. 10 -- Cambodia's four warring parties, marking what they called a historic first step toward peace after 20 years of conflict, agreed today to share power on a national council designed to rule Cambodia until United Nations-supervised elections can be held.

After a two-day meeting here co-sponsored by Indonesia and France, the Cambodian factions accepted a peace plan issued last month by the five permanent members of the U.N. Security Council. They also agreed on a 12-member body that is to represent Cambodia's sole "source of authority" during the transition period envisaged in the plan.

A three-page "joint statement" issued by the four parties -- the Vietnamese-backed government in Phnom Penh and three guerrilla groups joined in a coalition -- gave no specific details of a cease-fire and did not set a date for the first meeting of the Supreme National Council.

Nor did it spell out how or when the council is to assume power in Cambodia as the transitional authority, or under what circumstances it would cede some powers to the United Nations as called for in the Security Council plan.

Yet the accord marked the first time that the four have agreed to even a vaguely defined peace formula and to any kind of power-sharing arrangement. Although the document was not signed by any of the participants, the heads of the four Cambodian delegations appeared together at a joint press conference with the two co-chairmen of the peace talks tonight and endorsed the agreement.

The accord was reached by Hun Sen, the prime minister of the Communist government in Phnom Penh, and representatives of the three guerrilla groups: Khieu Samphan of the Khmer Rouge, which was responsible for the deaths of more than 1 million Cambodians during its rule in the 1970s; Prince Norodom Ranariddh, who represented his father Prince Norodom Sihanouk; and Son Sann, the head of the non-Communist Khmer People's National Liberation Front.

The Supreme National Council will be composed of six representatives of the Vietnamese-installed Hun Sen government that currently controls most of Cambodia, plus two members from each of the three guerrilla groups that make up a resistance coalition battling Phnom Penh for a decade, the parties agreed.

Designated as members of the Supreme National Council were the heads of the four Cambodian delegations at the talks here, five other representatives of the Phnom Penh government and three other members of the resistance groups.

Among the latter was Son Sen, the military commander of the Khmer Rouge and a close associate of Pol Pot, who led the Khmer Rouge during its bloody 3 1/2-year rule and is believed by some analysts to still play a role in the organization despite his announced retirement.

In deference to Sihanouk, the former Cambodian monarch and leader of the largest non-Communist resistance group, the four parties accepted his proposal to leave open the possibility of electing a "chairman" for the Supreme National Council as its 13th member.

Sihanouk, who boycotted the meeting in a tiff with Hun Sen and is considered likely to be named council chairman, announced qualified support for the panel from his exile home in Beijing, but said he was renouncing "political and diplomatic activities" for at least six months "because of ill health."

Ranariddh called the accord a "breakthrough" and a "crucial step forward" that was unimaginable only a few months ago. In an interview, he said his father had conveyed his "irrevocable" refusal to join the Supreme National Council as a member or as its chairman, but he shrugged when asked if the 68-year-old prince might change his mind.

The council "cannot be a supreme one without his {Sihanouk's} presence," Ranariddh said, adding that he would urge the body to "respectfully invite him to lead us."

Indonesian Foreign Minister Ali Alatas, who chaired the peace talks here along with French Foreign Ministry official Edwige Avice, hailed the accord as a remarkable achievement, but cautioned that much hard negotiating remains to be done before a final settlement can be reached.

"The road ahead of us may still be rough," he said, "but we are confident that we have entered a new political era."

The four parties, according to the pact, accepted the U.N. peace plan "in its entirety as the basis for settling the Cambodia conflict" and agreed to develop this U.N. "framework document" into a comprehensive political settlement in new negotiations under the Paris International Conference on Cambodia. The four also accepted "the nature and functions" of the Supreme National Council as stipulated in the U.N. plan.

Specifically, the accord said, that includes allowing the council to represent Cambodia at the United Nations and on other international bodies and turning over to U.N. authorities "all powers necessary to ensure the implementation of the comprehensive agreement."

The accord thus had the immediate effect of heading off a showdown when the United Nations convenes later this month. In a policy shift this summer, the United States said it would no longer support U.N. recognition of the resistance coalition as Cambodia's legal government since it includes the Khmer Rouge. It was not immediately clear, however, precisely who would take the U.N. seat, currently occupied by a Khmer Rouge representative.

The U.N. plan, issued by the United States, the Soviet Union, China, Britain and France, calls for the United Nations to assume unprecedented military and administrative powers in Cambodia from the Supreme National Council in a transition leading to free elections and a new constitution.

The plan says that during the transition, the length of which has yet to be determined, a three-stage disarmament process will take place in which the guerrilla groups will surrender their weapons to U.N. authorities. It also calls for the United Nations to "supervise and control . . . if necessary" the ministries of defense, foreign affairs, finance, public security and information until elections are held.

In a message sent from Beijing today, Sihanouk said he "supports the Supreme National Council as long as its decisions do not go against the real interests of Cambodia." He defined those interests as complete independence, the absence of any Vietnamese military or illegal civilian presence in Cambodia and a return to the borders of the country before Sihanouk was overthrown in a 1970 coup.

Since then, Cambodia has suffered through five years of war between the U.S.-backed Lon Nol government and the Khmer Rouge, nearly four years of destruction and terror under the radical Communist regime of Pol Pot and a Vietnamese invasion in December 1978 that installed the Hun Sen government.

In a joint press conference concluding the talks, Hun Sen evaded questions on whether he would accept Sihanouk's participation on the National Council.