A battlefield strewn with corpses and the burned hulks of Soviet-built equipment testified to a major clash between Angolan troops and rebel forces in southeastern Angola that reportedly has resulted in a government pullback from one of the major battles of the 10-year-old conflict.

Rebels of Jonas Savimbi's National Union for the Total Independence of Angola (UNITA) reported that they turned back the major assault just north of the key town of Mavinga and still 150 miles from UNITA's bush headquarters at Jamba, which Angola claimed 10 days ago he had abandoned in the face of their nearly two-month-old campaign.

This became apparent when a group of foreign correspondents visited the scene of battle along the banks of the Lomba River yesterday, and saw evidence of Angolan losses.

The corpses of Angolan soldiers and desG Angola has denied, and informed observers in Angola have given little credence to UNITA charges of active Soviet involvement in the recent fighting.

Matamba, who spoke Russian, said he had spent three years in the Soviet Union learning to fly MiGs. Appearing at the press briefing with his arm in a sling, he said he was shot down by UNITA antiaircraft cannon while flying a bombing raid on Mavinga on Oct. 3.

Questioned by the reporters, the pilot said he had not encountered any South African aircraft during the 45 missions he had flown during the campaign.

South Africa acknowledged publicly for the first time last month its "material, humanitarian and moral support" for UNITA.

Savimbi denied emphatically that he had received any active South African assistance. "There was not a single South African soldier here," he said. "We did not need it, we did not request it, and South Africa was not prepared to give it."

The retreating Angolan Army now reportedly is about 20 miles north of the Lomba River, withdrawing toward the town of Cuito Cuanavale. With Angola's tropical rainy season due to begin this month, making the sandy tracks through the bush impassable to trucks and armored vehicles, a counterattack seems unlikely.

Savimbi repeated his frequent appeals for the West to come to his assistance and said that since the latest assault on his position, bu far the biggest of the war, there were signs of growing support for him within the Reagan administration.

Savimbi said he believed Luanda's Soviet advisers had decided to launch this major conventional invasion of the UNITA-controlled sector of southeast Angola because it wanted to test the United States' willingness to come to his aid.

He said he believed the Soviets also wanted to take advantage of South Africa's racial unrest, testing Pretoria's willingness to become involved in Angola at this time and the West's reaction if it did.

A third reason, Savimbi suggested, was to counter a movement within Luanda's governing MPLA party that wanted to begin negotiations with UNITA.

The UNITA leader said the Angolan Army, which he said is commanded by Russian officers and supported by a substantial number of the estimated 25,000 Cuban troops in Angola, had launched the campaign on two fronts from the central town of Menongue on Aug. 15.

One column, using Russian T34, T55 and the latest T62 tanks, moved east into the Cazombo salient that juts into neighboring Zaire. It gained control of the town of Cazombo, and the plan was presumably for it to turn southward later, linking up with the other column, Savimbi said.

The second column moved southeast to Cuito Cuanavale, where it split into two prongs making a pincer movement toward Mavinga. This constituted a direct threat to the UNITA stronghold at Jamba, 150 miles farther south and close to the Namibian border.

Savimbi said the two pincers of the southern attack came together on the south bank of the Lomba River Sept. 7. By that time UNITA had managed to concentrate its forces in the area between Mavinga and the river to stop it.

UNITA launched a counterattack Sept. 26, and after three days of heavy fighting the turning point came Sept. 29 when the Soviet commanders decided to withdraw across the Lomba, he said.

The visit to the Lomba River battlefield provided close-up impressions of a major campaign that has been going on for nearly two months but has been reported only through contradictory statements form Luanda, UNITA and Pretoria.

The reporters flew in a chartered aircraft to a bush landing strip at Mavinga, their 42-year-old Dakota crossing at night and dropping to treetop height in the dark to come in below the Angolan radar network.

After the reporters spent the night in underground bunkers, a UNITA patrol took them in trucks to the Lomba River battlefield.

The number of unburied corpses, and a group of 20 heavy Russian Zil troop carriers -- one with a multiple rocket launcher mounted on it -- that appeared to have been destroyed in a single attack, suggested that at least some of the Angolans had been caught by surprise.

The Luanda government claims there were three South African air attacks on their troops, but Savimbi denied this.

What seems more likely, according to some western military analysts in pretoria, is that South Africa may have sent a unit of black Portuguese-speaking commandos to Jamba to release UNITA fighters from their headquarters.

Savimbi gave Angolan casualties in the battle as 2,300 killed and wounded. He said 410 UNITA troops had been killed and 832 wounded. Nine Russians and 38 Cubans were killed, he added