CUNJAMBA, ANGOLA -- Anticommunist rebels have blunted a major government offensive against their strongholds in southern Angola but are bracing for a second, more massive assault that could determine the outcome of the 12-year-old civil war, according to guerrilla leader Jonas Savimbi.

Savimbi, who receives U.S. and South African support, said his 8,000 guerrillas face two columns of at least 18,200 Soviet-directed government troops along the vital Lomba River. He predicted that before the rainy season begins in the next three weeks, "There will be thousands and thousands of bodies in Lomba."

Savimbi, leader of the National Union for the Total Independence of Angola (UNITA), spoke with reporters at front-line positions near the Lomba River and at a news conference at his bunker headquarters in Mavinga, 30 miles southeast of here.

The expected battle would be decisive, Savimbi said, because the Angolans have committed their best troops to the two columns, backed by 150 tanks and more than 200 armored vehicles.

He said that UNITA is prepared to bring up reserves to match the enemy strength and that he was relying on U.S.-supplied antiaircraft missiles to limit the effectiveness of air support by Cuban- and Angolan-piloted MiG23s and MiG21s.

Savimbi said that a "spectacular" battle had taken place on the Lomba River near here on Sept. 13, when UNITA forces drove back two battalions of government troops who had fought their way to the south side of the waterway and encamped for a week.

However, casualty figures given by his aides suggested a battle of less ferocious proportions had taken place. They said that while three government battalions had been routed and three tanks destroyed, only 70 dead government soldiers had been counted, compared to 10 UNITA dead.

UNITA claims that since the first tentative probes of the current dry season offensive began three months ago, 1,023 government troops have died in fighting, compared to 86 UNITA dead. It also claims to have destroyed 56 tanks and shot down 11 government helicopters and three MiG fighters.

Two Soviet military personnel were killed and four injured in recent fighting, UNITA intelligence officers said. They offered no evidence to substantiate their claim.

In an attempt to prove that UNITA forces had stemmed a major assault near here and that another was imminent, Savimbi on Friday took a group of South African-based correspondents by plane, truck and foot to a point 12 miles north of the river in an area defended by camouflaged infantry, artillery and antitank positions.

There, UNITA chief of staff Gen. Ben Ben pointed across a shallow valley two miles to the north, which he said was the front line of the battle where several thousand government troops were dug in. But the reporters could see no evidence of Angolan troops from that distance and no sign of the Sept. 13 fighting.

For the benefit of photographers, UNITA guerrillas fired off salvos of rockets in the direction of the government positions, but there was no reply.

Later, two MiG fighter-bombers were seen making a bombing run near Mavinga, sending up spirals of black smoke that appeared to be from burning oil.

Ben said that in the battle three weeks ago, UNITA forces routed government infantry units that advanced ahead of Soviet-made T55 and T54 tanks, leaving the tanks vulnerable to rocket and antitank missile fire as they approached the UNITA defensive positions.

Ben and Savimbi denied reports by the Angolan government that South African air and ground forces were involved in the fighting on UNITA's side.

South African Defense Minister Magnus Malan was quoted by the state-run South African Broadcasting Corp. last night as confirming that South Africa maintains a presence in southern Angola, although he did not explicitly confirm that the forces have been used against Angolan troops in the current offensive.

Malan, the broadcast said, pledged South Africa's continued "material and humanitarian" support of UNITA, saying the rebels are in the forefront of blocking Soviet expansionism in Angola.

UNITA intelligence chief Brig. Peregrino Chindondo played a tape recording of what he said were Soviet pilots speaking with ground observers during an air-support mission along the Lomba River.

In his bunker headquarters near Mavinga, Savimbi said he expects that before the dry season ends, government forces will mount major offensives against UNITA positions near Chambinga, about 13 miles east of Cuito Cuanavale, and again against the defensive line near here.

On July 30, in an interview in Mavinga, Savimbi predicted a final, decisive government assault against Chambinga would begin imminently.

On Friday, the UNITA leader told reporters that the outcome of the expected battles, which he predicted would be the largest in his 12-year-old campaign to overthrow the Marxist government in the capital, Luanda, "will affect southern Africa for years to come.

"This will be the last offensive. If they lose, the Soviets will have to take stock of lessons learned and then negotiate with UNITA. If UNITA is wiped out, the Russians will turn Angola into a base for their next offensive in the region," Savimbi said.

He added, "We never said we beat the offensive. We say we have the upper hand of the situation, and that we can determine the fate of these brigades on the Lomba."

Savimbi said the expected government thrust against Chambinga will be aimed at driving toward Mavinga, which is strategically important not only because it is the gateway to UNITA's main headquarters in Jamba, 170 miles to the southeast, but because it produces virtually all of the food for the UNITA-controlled southeast corner of Angola.

In each of the past several years, government forces have tried to take Mavinga and have been repulsed. In 1985, government troops approached Mavinga but were stopped when South Africa intervened with massive air support.

Savimbi said he was not anticipating a military solution to the civil war, because there is no possibility for one by either side. Instead, he said, he is aiming at a major victory either here or at Chambinga that will force the government of President Eduardo dos Santos to negotiate a settlement with UNITA.

He argued that a power-sharing agreement between UNITA and the ruling Popular Movement for the Liberation of Angola would weaken Soviet and Cuban influence in Angola and remove obstacles to independence for neighboring Namibia (South West Africa).