The Angolan government's news agency reported today that rebel leader Jonas Savimbi had abandoned his base at Jamba in the southern part of the country and withdrawn into neighboring Namibia following "an important military victory" by the Angolan Army.

South Africa, which supports Savimbi and has called the two-month-old Angolan offensive against the guerrillas a threat to its regional security interests, "categorically denied" the report.

The news agency, Angop, identified no specific action against the guerrilla stronghold at Jamba by the Angolan Army, which has been waging its biggest ever offensive against Savimbi's National Union for the Total Independence of Angola (UNITA) in southeastern Angola during the past two months.

There was no independent confirmation of the Angolan report.

Figueiredo Paulo, a UNITA representative in Washington, said, "The situation remains as it was. Jamba is not threatened."

The loss of Jamba would be a heavy blow to Savimbi's movement. Jamba is a sprawling and highly sophisticated bush camp about 100 miles north of the Namibian border with accommodations for a population of about 10,000. UNITA has staged several press conferences there for foreign journalists.

Fuel and other supplies have been brought across the border from Namibia, where the South African Army has bases for its counterinsurgency war against the Namibian independence movement, the South-West Africa People's Organization (SWAPO).

It has been evident for more than a week that Jamba was under threat. The Angolan Army, with stepped-up Soviet support, has been pushing toward it in a two-pronged thrust from a newly established base at Menongue, in south-central Angola. These columns have been strengthened by Soviet tanks, as well as air support from big Mi24 helicopter gunships.

Faced with this threat to its ally, Pretoria has become more open in its longstanding support for UNITA. Defense Minister Magnus Malan admitted South Africa's aid to Savimbi last Sunday and warned Monday that it was prepared to intervene militarily to save him.

It is an open secret in Pretoria that South African jets carried out two strafing raids on the advancing Angolan columns, as Angop reported last week, and that commandos of South Africa's 32 Battalion, made up mostly of Portuguese-speaking blacks, moved into Jamba alongside Savimbi's men.

Angop said today that the South African raids had failed to check the Angolan advance.

In its latest campaign against the guerrillas, the Angolan Army has relied on a massive Soviet military build-up, which began after South Africa's big "Operation Askari" invasion of Angola in January 1984. In that operation, South African troops clashed directly with the Angolan Army and their Cuban supporters, according to the South African government.

It was widely reported in the South African press and confirmed by the Foreign Ministry at the time that a secret meeting had taken place late in 1983 near the United Nations in New York, at which Russian diplomats warned South African representatives that Moscow might step up its support for Angola, with which it has a friendship treaty, if Pretoria continued to threaten Angola and support UNITA. There are about 25,000 Cuban troops stationed in Angola along with Soviet and East European advisers.

Since the 1984 invasion, the Soviets have installed a sophisticated air defense system, with radar cover and ground-to-air missiles. They have equipped the Angolan Army with a substantial number of tanks and the Mi24 helicopter gunships, which a western military specialist here described as "perhaps the most formidable combat helicopter in the world today."

Finally, the Russians lengthened the airstrip at Menongue, bringing southern Angola within range of their Sukhoi bombers and MiG fighters. This has shifted the balance of power in Angola. Only six months ago, UNITA guerrillas were carrying out raids around Luanda and Savimbi was predicting he would soon enter the capital.

The reversal poses severe problems for South Africa because UNITA's control over southeastern Angola effectively has sealed off half the long border with Namibia to infiltration by SWAPO insurgents.