JOHANNESBURG, SEPT. 29 -- The South African Defense Forces refused to comment substantively today on persistent reports that South Africa helped repulse a major offensive by Soviet-backed Angolan troops against anticommunist UNITA rebels earlier this month.

A military spokesman in Pretoria characterized as "contradictory" reports in progovernment Afrikaans newspapers today that South Africa was "definitely" involved in defending UNITA during a counteroffensive against Angolan troops in southern Angola two weeks ago.

U.S.-supplied forces of UNITA, which in Portuguese stands for the National Union for the Total Independence of Angola, recaptured the Lomba logistics base near the town of Mavinga in a battle that began on Sept. 13 and killed an estimated 350 Angolan soldiers, according to UNITA commanders.

The battle, which was said to involve seven brigades totaling an estimated 10,000 Angolan troops, has been described by UNITA as the biggest since the dry-season offensive of 1985, when Angolan armored and infantry columns pushed eastward from Cuito Cuanavale toward Mavinga until South Africa intervened with massive air support.

Not only is Mavinga strategically important as a gateway to the rebels' bush headquarters in Jamba, near the Namibian border, but the area produces virtually all of the food for the UNITA-controlled southeastern corner of the country.

The Angolans, backed by an estimated 37,000 Cuban troops guarding strategic positions to the north, intend to try to capture Mavinga as a jump-off point for an assault on Jamba next year, according to UNITA commanders. Jamba is 170 miles southeast of Mavinga.

A second force of an estimated five brigades was reported to be moving westward from the Cangamba area toward Tempue in an effort to block routes used by UNITA guerrillas to infiltrate northward to strike the Benguela railway, diamond mines and agricultural centers.

Military analysts here said the purpose appears to be to force UNITA to split its estimated 28,000 troops between the two fronts.

U.S. intelligence agencies estimate that the Soviet Union has sent $1 billion worth of arms to Angola in the past year in preparation for the current offensive. Much of it has been flown south from the capital of Luanda to Cuito Cuanavale.

Afrikaans-language newspapers here, quoting "informed military spokesmen," said the South African presence in Angola should be viewed against the necessity to protect South Africa's interests by assuring that UNITA is not defeated.

UNITA long has been engaged in containing guerrillas of the South West Africa People's Organization (SWAPO) who seek to cross the border into Namibia, which South Africa controls.

Defense Forces spokesman Commandant Ian Buck today refused to go beyond an earlier Army statement that "this report {of intervention} confirms once again that the major offensive against UNITA is Russian- and Cuban-inspired, controlled and commanded."

He was referring to reports in the British press that South African Mirage fighter-bombers were sent into action earlier this month to save Mavinga from capture by Angolan forces. Mavinga, where Savimbi maintains his command post, has an airstrip used to supply UNITA forces at the front east of Cuito Cuanavale.

The Angolan offensive is said to have been directed by Soviet Gen. Konstantin Shagnovitch, described by UNITA as the highest ranking Soviet officer assigned to a foreign military command outside of Europe and Afghanistan.

The London Sunday Telegraph reported that two Angolan brigades started crossing the headwaters of the Lomba River, 60 miles west of Mavinga, before they were attacked by South African Mirage-3 aircraft. A South African photographer who visited the Lomba battleground, Cloete Breytenbach, said he had seen the burned wreckage of three Soviet-made T55 tanks, two bridge-laying vehicles, a rocket launcher and armored vehicles.

South African Defense Minister Magnus Malan, in a speech to Parliament earlier this month, made it clear that South Africa did not intend to let UNITA collapse under the Angolan offensive.

Malan said it was South Africa's duty to "protect the region against Russian and Cuban destabilization and objectives of eventual takeover.

"If our interests demand that we take action against communist incursion, then we have no choice."

Savimbi, in an interview in Mavinga last month, said that the expected offensive would be a "life or death" battle for UNITA.