JOHANNESBURG, South Africa (CNN) -- Angolan refugees were pouring into the town of Kuito on Tuesday, fleeing a 10-day UNITA rebel offensive that was cutting through the country's central highlands.

Relief workers said Kuito, 600 kilometers (375 miles) southeast of the capital Luanda, was virtually cut off by a rebel siege from the rest of the country.

"We have the makings of a disaster on our hands," said a U.N. official who asked not to be named. "Kuito has been completely cut off. We cannot get in there by road or air."

The official said as many as 9,000 displaced people had entered Kuito in the last few days, raising the number of Angolan refugees there to 80,000 since the fighting began in early December.

Angola has labored under an uneasy peace since a 1994 U.N.-brokered deal ended a 20-year civil war. Full implementation of the accord was stymied, because the government and the rebels continue to harbor deep suspicions of one another.

The Angolan government charged that the rebels were not abiding by the terms of the 1994 deal, and that the United Nations had failed to hold UNITA to its agreements.

"(The United Nations Observer Mission) is not doing anything here," said Angolan Minister for Territorial Administration Faustino Muteka. "The (UNITA) men being demobilized were not soldiers, the weapons handed over were obsolete and UNITA was still training troops and receiving new weaponry."

Analysts say UNITA has proved a stronger foe for government forces -- with funds obtained through illegal diamond mines.

UNITA -- a Portuguese acronym for the National Union for the Total Independence of Angola -- has refused to hand over its central highland strongholds to the government.

The current fighting erupted after government forces launched an air and land offensive against the UNITA strongholds of Andulo and Bailundo.

UNITA launched its own attack, taking eight towns in the central provinces and threatening Kuito and the town of Huambo, 70 miles (113 km) west of Kuito.

A senior Catholic church official in Kuito said fighting in the immediate area was "sporadic" and not heavy. "But it can still become dangerous," he said.

Relief workers in Kuito and Huambo have been evacuated.