Angolan guerrilla leader Jonas Savimbi conceded today that his rebel forces were under severe pressure from a two-month-old government offensive but denied that his main base was on the verge of being captured.

Savimbi dismissed Angola's claims that its troops were converging on headquarters of his National Union for the Total Independence of Angola (UNITA) at Jamba. He denied that the South African troops that entered southern Angola earlier this week had intervened on his behalf. He said his men were holding off government forces more than 125 miles to the north of the rebel base.

In his first public statement since the offensive began, the rebel leader said at a midnight press conference in Jamba that government forces were using increasingly sophisticated, Soviet-made weaponry in the offensive, including Mi25 helicopter gunships, MiG21 and 23 warplanes and T62 tanks, according to reports from South African journalists who attended the briefing.

All of these weapons were manned by Soviets, Savimbi claimed, and for the first time, Soviet military officers were participating directly in the fighting as tactical commanders.

"We have had offensives before, but never have we had Russians planning and directing artillery, tanks and armored cars and flying helicopters and jets against us," Savimbi told reporters.

The Angolan government has denied, and informed diplomatic observers give little credence to, UNITA charges of Soviet involvement in recent fighting, Washington Post correspondent Karen DeYoung reported last week from Luanda, the Angolan capital.

Savimbi's forces have been battered by a nearly two-month-long offensive by government forces backed by Soviet and Cuban military advisers. Luanda is seeking to establish control over the southeastern third of Angola that Savimbi, backed by South African arms and logistical support, long has ruled.

Savimbi said he ordered the evacuation yesterday morning of the town of Cazongo, a major logistical base, after Soviet warplanes napalmed civilians, agricultural installations and hospitals there.

Savimbi also said that heavy fighting was continuing outside Mavinga, 125 miles north of Jamba, but that government forces had been stopped about 15 miles north of the town.

Savimbi said his forces had killed nearly 1,200 Angolan troops plus three Soviet and 18 Cuban advisers since the offensive began in late July. He said his forces had lost only 150 men killed and 630 wounded. Angola has claimed to have killed nearly 1,000 rebels. There is no independent confirmation of either claim.

Reporters had been flown to Jamba to prove that the rebel base was still firmly under UNITA control, Savimbi said, but he hinted that the rebels might abandon the base if it is attacked.

Meanwhile, scattered incidents of unrest were reported by police in more than half a dozen black townships in South Africa. One man died overnight in a township outside Port Elizabeth when his house was set on fire. Police had no further details of the incident.