JAKARTA, INDONESIA, SEPT. 9 -- Cambodia's four warring factions today opened a new round of peace talks and endorsed a plan worked out by the five permanent members of the U.N. Security Council as the framework for a solution to the country's 11-year conflict, participants said.

Significant obstacles to peace remained, however, including differences over the composition of a "Supreme National Council" that would rule Cambodia until free elections could be held.

The U.S. ambassador to Indonesia conferred this afternoon with Cambodian Prime Minister Hun Sen -- the first meeting by a U.S. administration official with the head of a Communist government the United States long has refused to recognize. The U.S. considers it a "puppet regime" installed after the 1978 Vietnamese invasion of Cambodia. Secretary of State James A. Baker III last week announced that, following a similar opening toward Vietnam, Washington would start talks with the Hun Sen government on a Cambodian settlement.

Hun Sen called the half-hour meeting with Ambassador John Monjo "very valuable" and said the American had sought his views on the U.N. Security Council plan and on prospects for a solution in Cambodia. Monjo described the meeting, which took place in Hun Sen's room at the hotel where the peace talks are being held, as "just an exchange of views," and the ambassador said he had no message to Hun Sen from President Bush.

In a press briefing afterward, Hun Sen reiterated his acceptance of the U.N. plan as the "framework" for a settlement, but he dodged questions on whether he still has reservations about it.

Later, however, Indonesian Foreign Minister Ali Alatas, who is a co-chairman of the conference along with a French representative, told reporters that all four Cambodian factions "have indicated they accepted {the U.N. plan} as the basis for settling the conflict." He said Hun Sen has no reservations about the plan.

Alatas, who has been holding separate talks with each faction in an effort to hammer out an agreement, said the four would start discussions Monday on the formation of a Supreme National Council as called for in the plan issued last month by the United States, the Soviet Union, China, Britain and France.

The council, serving as the highest Cambodian governmental body, is to turn over powers to an unprecedented U.N. military and administrative presence in the country under the plan.

On Saturday, non-Communist resistance leader Prince Norodom Sihanouk said the three guerrilla groups -- the Communist Khmer Rouge, Sihanouk's group and another non-Communist group -- now accept a formula that would place on the council six representatives from the rebel coalition and six from the Phnom Penh government, plus a "chairman," likely to be Sihanouk.

Under this formula, each guerrilla faction would have two seats while Hun Sen's government would have six. The resistance, notably the Khmer Rouge, previously had demanded an equal number of seats for each of the four factions.

But today, Phnom Penh officials said they were reluctant to accept the "chairman" as a 13th member, insisting on only 12 members, Reuter reported.

According to Asian diplomats, talks between Vietnam and China also have been taking place here in an effort to cement an accord.

China, Vietnam's archenemy, has strongly supported the Khmer Rouge, the most powerful member of the resistance coalition, in the struggle against the government in Phnom Penh.

That government was installed after Vietnamese troops invaded Cambodia in December 1978 and ousted the Khmer Rouge, which was responsible for the deaths of more than 1 million Cambodians during its 3 1/2-year rule.

Vietnam, which kept a large occupation force in Cambodia until last year, was assisted in its war effort through most of the 1980s by the Soviet Union, but Moscow has been pressing for a settlement in the last couple of years.

Given the unprecedented big-power consensus on Cambodia, Alatas told the four factions in opening the talks today, "the circumstances for achieving a comprehensive settlement are unlikely ever again to be as favorable as they are now." He called on the four to "seize the unique opportunity" and put aside their "mutual mistrust and enmity."

"Anyone or any party seen to be blocking the path to peace will have to assume an awesome responsibility before history," Alatas warned.