LUANDA, Angola (AP) _ A top army officer today warned Angolans to prepare for a protracted war against rebels, prompting relief workers to appeal for a cease-fire so they can get aid to civilians trapped behind battle lines.

The government's chief of staff, Joao de Matos, said the government was preparing for ''a war of great duration'' following the rebels' failure to show up for peace talks last week in Ethiopia, the Portuguese news agency LUSA reported.

In response, the World Food Program has ordered a new, larger capacity cargo plane and is increasing supply runs to the estimated 3 million Angolans trapped by the fighting, said Pierre Honnorat, the group's logistics coordinator.

''But a cease-fire means everything,'' Honnorat said. ''The two most important areas are Huambo and Kuito, and it is impossible to fly there.''

The rebel group, the National Union for the Total Independence of Angola, has repeatedly refused a cease-fire to permit an emergency airlift to civilians trapped for two months in Huambo, 329 miles southeast of Luanda.

Huambo, once Angola's second-largest city with a population of 400,000, has become the key battle zone in the renewed civil war since government troops tried to overrun the rebel headquarters there on Jan. 9.

Peace accords signed between the rebels and the government in May 1991 to end a 16-year war called for the rebel army to demobilize and a unified military to be created. But many rebel forces withdrew to the bush rather than surrender their arms.

Fighting broke out again on Oct. 31 when rebel leader Jonas Savimbi refused to accept defeat in the country's first multiparty elections a month earlier.

His forces since have seized control of three-quarters of the country.

The civil war that began in 1975 has destroyed the agricultural base of the once fertile southern African nation, and relief workers warn say that continued fighting will cause a tragedy on the scale of Somalia.

The World Food Program has about 16,000 tons of food stockpiled, Honnorat said, far short of the 24,000 tons needed each month to feed people in the drought and civil-war devastated country.

But the most difficult problem is transporting the maize, vegetable oil and dried fish over roads mined by rebel troops and skies guarded by rebel anti- aircraft guns.

The relief agency increased flights from two to four a week to sustain 90,000 people trapped in the desert-side city of Luena, 434 miles east of Luanda, Honnorat said.

''In Luena, we had to stop flights for 15 days because of the fighting, and when we got there again, people were quite desperate,'' he said.