CAUGHT for years in the crossfire of their country's civil war, Cambodian refugees in Thailand fear new repression by the Khmer Rouge even as the United Nations begins an unprecedented repatriation effort.The recent replacement of 16 civilian administrators with hard-line military leaders at a showcase border camp controlled by the Khmer Rouge worries UN officials and Western aid workers preparing to return more than 350,000 refugees to Cambodia. The Marxist Khmer Rouge, who are blamed for the deaths of more than 1 million Cambodians during the 1970s, have already been trucking people across the Thai border under cover of darkness to establish a power base before UN-supervised elections, international aid workers say. To block more movements this weekend at the Khmer Rouge Site 8 camp, UN officials have asked the Thai government to increase troops near the camps; they say refugees could use violence to resist the guerrillas. "People are frightened because they want to go back under the auspices of the UN," says a UN border official about Site 8, which has 43,000 refugees, two-thirds of those in Khmer Rouge-controlled camps. "It's very tense." Less than a week before the signing of a peace accord ending Cambodia's 12-year civil war, the UN's biggest repatriation ever is clouded by forced resettlement, funding and logistical problems, and threats from disease and land mines. (UN peacekeepers; bare-knuckle border trade, Page 7.) Along the border, UN officials fear anarchy is replacing war. The Communist regime in Phnom Penh and the three-party resistance declared an indefinite cease-fire last June. But as fighting slowed, attacks, banditry, and killings have increased among rival guerrilla soldiers and between the Phnom Penh Army and police, all left jobless and impoverished by peace. For years, rivalry and indiscipline has been rife among the rival armies. Now amid the cutoff in foreign military supplies, unpaid soldiers are selling their weapons and scrambling to support their families. The cross-border market has responded, buying and selling weapons made cheap by desperation. In late August, dozens of bandits rampaged through Site 2, firing automatic weapons and mortars, taking hostages, and plundering. Recently the camp was attacked again. "It's the wild West out there," says Robert Burrows, a UN official in Aranyaprathet. "This could cause serious problems for repatriation." For Noun Navy, peace is a bewildering prospect . More than a decade ago, Mrs. Noun and her husband fled Cambodia's spreading civil war for a refugee camp in Thailand. But with an agreement and the end of her exile near, the 31-year old woman is uneasy. Her husband long ago deserted her, disappearing from the bamboo and thatch refugee labyrinth at Site 2 where more than 200,000 Cambodian refugees live. She recently reestablished contact with her parents inside Cambodia but is not eager to go home. "I am afraid and will wait to go with everyone else," says Noun. Complicating the UN repatriation plan is a lack of funds, UN officials say. Preparations, including the designation of transit camps and construction of six reception centers in Phnom Penh-controlled areas, is expected to cost $33 million. A total $100 million will be needed to complete the operation. Only $9 million has been pledged so far, mainly by Japan, which is expected to bankroll much of the repatriation. Cambodian observers and Western aid workers fear the massive operation by the UN, which will administer Cambodia in the run-up to a national election, will overwhelm a shattered and corrupt country unprepared to absorb huge amounts of assistance. UN officials say careful scrutiny will be crucial. "There had better be good monitoring," says Burrows. "Otherwise, it will go in one door and out the back." "Cambodia is rotten from top to bottom,' says a veteran Western aid worker. "With the West feeling guilty and pouring money in, it is one of the big problems for the future." The United Nations High Commissioner for Refugees (UNHCR) predicts that 10,000 Cambodians a week can be escorted home, beginning at the dry winter months next year. At that rate, the camps can be emptied in six to nine months. Registration of the refugees is well under way as well as a poster campaign in the camps urging the Cambodians to be patient and wait for UN repatriation. Pointing to conflicting pressures, however, officials say it's difficult to predict the refugees' likely response. Many Cambodians are expected to return when the country's exiled monarch, Prince Norodom Sihanouk, goes home in mid-November as the head of the interim Supreme National Council. Still, in recent months, many Cambodians have been daunted by fear of land mines, food shortages, spreading malaria, and the country's worst flooding in 50 years. "The reintegration of this border population will be a big problem," says Jean-Jacques Fresard, an official with the International Committee of the Red Cross in Phnom Penh. "They will be foreigners in their own country."