PHNOM PENH, Cambodia (AP) _ The Khmer Rouge guerrillas apparently broke with Cambodia's peace process Tuesday, U.N. sources said, raising fears that months of mounting tensions will reignite civil war.

The sources said all Khmer Rouge officials in the capital appeared to have left Tuesday and the group sent a letter of withdrawal to U.N. officials.

The sources said the letter did not indicate whether the Khmer Rouge was severing all relations with the U.N. mission that has been trying to implement a cease-fire and hold elections next month under a 1991 peace accord.

Relations between the guerrillas and United Nations have grown worse over the past year, feeding fears that full-scale war could resume. U.N. officials have blamed the Khmer Rouge for several recent attacks on U.N. personnel and said the group was suspected in most of the rest.

The Khmer Rouge denied responsibility for the attacks, and its leader, Khieu Samphan, warned U.N. officials Saturday that it was ''dangerous'' to accuse his group.

Officials of the Vietnamese-installed government could not be located because of the three-day Cambodian new year that began Tuesday. The capital was eerily silent, save for scattered fireworks and gunshots.

Khmer Rouge delegates, who set up offices in the capital more than a year ago for liaison with U.N. peacekeepers, did not answer their telephone Tuesday night. Guards posted by the government outside the compound said it was empty.

U.N. officials, who spoke on condition of anonymity, said the dozen or so Khmer Rouge personnel apparently evacuated the compound earlier in the day and flew to Bangkok, Thailand. From there they could go by land to the Khmer Rouge stronghold of Pailin in northwestern Cambodia along the Thai border.

A U.N. spokesman confirmed Tuesday night that U.N. officials received a copy of a letter from the Khmer Rouge to Prince Norodom Sihanouk. Sihanouk is chairman of a reconciliation council set up under the peace accord signed by Cambodia's government, the Khmer Rouge and two other rebel factions.

The spokesman, Eric Berman, said U.N. officials were trying to verify the letter's authenticity and determine what it meant.

One source in the U.N. mission said the government of Premier Hun Sen had been pressing peacekeepers to arrest Khieu, the Khmer Rouge leader, for recent attacks on U.N. personnel and ethnic Vietnamese.

Seven U.N. peacekeepers and aid workers have been killed, and slayings of dozens of ethnic Vietnamese civilians triggered an exodus by thousands of Vietnamese to Vietnam.

Government forces control most of Cambodia's territory, including the capital, but most Cambodians are terrified of the Khmer Rouge.

The fanatical group seized control of the country during the 1975 new year and begin a radical effort to create an austere agrarian society. Hundreds of thousands of people were tortured, starved and worked to death until the regime was toppled in late 1978 after Vietnam's army invaded.

The Khmer Rouge retreated to the jungles along the Thai border and fought the government installed by Vietnam until the peace accord was signed in late 1991.