A government statement said Unita attacks were spreading death and destruction and forcing thousands of civilians to flee their homes.

Earlier, Unita fighters were reported to have launched a major attack on the strategically-important city of Cuito.

A Unita spokesman said it had been forced to act after the government launched an offensive earlier this month.

Correspondents say Unita appears to have seized significant territory in the latest fighting because it has developed a mobile war capacity combining armour and artillery.

Aid operation

Greg Barrow: The humanitarian consequences are potentially disasterous

The UN food agency, the World Food Programme, announced the start of an airlift operation into nearby Huambo to provide food for tens of thousands of people displaced by fighting around Cuito. The WFP said three plane-loads of food had been sent as the first of 18 flights planned over the next two weeks.

Huambo, the former Unita headquarters, was captured by the government in 1992. Cuito in neighbouring Bie Province provides a strategic gateway to the east and central highlands of the country.

The WFP estimates more than 200,000 civilians have been displaced since the resumption of fighting. A spokeswoman said most had fled with nothing.

Shortage of food

With no signs of the conflict ending, the United Nations Food and Agriculture Organisation has warned the country could be left seriously short of food.

Land mines continue to claim victims

The organisation said renewed fighting in Angola meant large areas of the country would need food aid in coming months, despite improvements in food production.

Last week there were unconfirmed reports of a massacre of civilians near Cuito prompting the government to appeal for calm.

Fierce fighting between Unita and government troops erupted on 5 December with heavy battles taking place for control of Cuito. At one point last week Unita rebels surrounded the city, which is 600km (375 miles) southeast of Luanda.

A Unita official was quoted as saying last week that the rebel group intended to continue its advance on both Cuito and Huambo.

Meanwhile, Zimbabwe and Namibia have dismissed as "pure propaganda" Unita suggestions that they have intervened on the side of the government in Angola.