The move sharpened conflicts among Cambodia's bitter rivals and further jeopardised the outcome of the country's first multi-party elections in two decades.
Yasushi Akashi, the UN chief in Cambodia, agreed to withdraw non-essential staff from the three key provinces of Kompong Cham, Svay Rieng and Prey Veng after Prince Sihanouk urged them to leave, saying he would not be responsible for their safety if they stayed. Military staff will remain.
Eric Falt, a UN spokesman, said later the mission had decided it would also withdraw non-essential staff from the other four provinces as a precaution.
The provinces in the so-called 'King Father Autonomous Zone', named for Sihanouk, are all located on the frontiers with Vietnam and Laos, and cover about 40 per cent of Cambodia.
The leader of the secessionist movement is Sihanouk's son, Prince Norodom Chakrapong. He told a rally of about 3,000 people in Svay Rieng that he had formed the zone in response to their objections about irregularities in last month's general election, which his party lost.
He said he wanted to unite Cambodians in support of his father, but admitted to journalists that he had not received Prince Sihanouk's approval.
Prince 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 won the election by a slim margin.
The rebel forces had barred Funcinpec and UN personnel from the zone and threatened UN police at gunpoint, leading them to abandon posts, Mr Falt said.
Funcinpec alleged rebels had killed dozens of its officials and members in the past few days in at least one province. The claim could not be confirmed.
Ung Huot, Funcinpec's election manager, said there were rumours that a coup would be staged tomorrow, and UN police were regularly patrolling the party's headquarters in the capital, Phnom Penh.
He said government forces were expected to seize the new assembly when it meets for the first time tomorrow, and kidnap Sihanouk and Ranariddh.
Mr Falt said Hun Sen, the defeated Prime Minister, threatened UN officials on national radio on Friday, saying he could not be held responsible for the 'destruction' of the UN radio station if it did not stop broadcasts against the government.
But Khieu Kanharith, a government spokesman, said Hun Sen had urged senior party officials yesterday to reject the autonomous zone and had issued a plea for peace.