In a major power grab, a wayward son of Prince Norodom Sihanouk declared autonomy for about 40 percent of the nation Saturday and demanded all U.N. personnel leave the region.
The move sharpened conflicts among Cambodia's already bitter rivals and further jeopardized the outcome of the country's first multiparty elections in two decades.
The U.N. said it would pull civilian staff from the seven eastern provinces controlled by forces loyal to Prince Norodom Chakrapong. But Yasushi Akashi, the U.N. chief in Cambodia, said: "The military will stay."
U.N. spokesman Eric Falt said the harassment by Chakrapong's forces, a renegade element of the Phnom Penh government, had been reported to the U.N. Security Council. "The international community is sure to react to further provocation," he said.
The provinces in the so-called "King Father Autonomous Zone" are situated along the borders with Vietnam and Laos. The name of the zone refers to Sihanouk, a former ruler and now the country's key reconciliation leader.
The move did not have public endorsement of the Vietnam-backed government, but its Prime Minister Hun Sen also threatened the United Nations.
Chakrapong told a rally of about 3,000 people in Svay Rieng that he formed the zone in response to charges of irregularities in the election for an assembly to draft a new constitution.
U.N. observers and independent monitors declared the elections free and fair, and many people at the rally said they did not support Chakrapong.
Chakrapong said he wanted to unite Cambodians in support of his father, but conceded to journalists that he had not received Sihanouk's approval.
Sihanouk has called for a united country under the leadership of another son, Prince Norodom Ranariddh, who is Chakrapong's half-brother. Ranariddh's FUNCINPEC party beat the government by a slim margin in last month's election.
"I don't think we are going to back down," said Ung Huot, FUNCINPEC's election manager. "The verdict of the Cambodian people should be implemented."
The rebel forces have barred FUNCINPEC and U.N. personnel from the zone. They also have threatened U.N. police at gunpoint, forcing them to abandon several posts, Falt said.
FUNCINPEC alleged the forces had killed dozens of its officials and members in the past few days in at least one province. The claim could not be confirmed.
"If they did not leave they would probably be dead by now," Ung Huot said. "It sounds like April 1975 when the Khmer Rouge took over Phnom Penh and the other provinces."
The Khmer Rouge killed hundreds of thousands of Cambodians after seizing control of the country in 1975. It was ousted when Vietnam invaded in late 1978, beginning 13 years of war.
Cambodia's four warring factions signed a 1991 accord to end the fighting and authorize the elections. The Khmer Rouge boycotted the poll but now is urging the government to honor the results.
Although Hun Sen and more moderate government officials have promised that threats against U.N. peacekeepers would not spread to the capital of Phnom Penh, U.N. officials said Hun Sen threatened them Friday on national radio.