Zimabwe-Rhodesia's war against black nationalist guerrillas is being supported directly by South African troops and warplanes and indirectly by Israel, according to an American who fought in the Rhodesian army.

William R. Atkins, 35, a former Prince George's County policeman and Vietnam War veteran who was expelled from Zimbabwe-Rhodesia last month, says that South Africa maintains as many as two combat battalions of paratroopers and at least one squadron of Mirage fighter-bombers in Zimbabwe-rhodesia. He asserts that South African forces, wearing Rhodesian uniforms and using equipment painted in the Rhodesian colors, effectively contrfol the southern quarter of the country.

Atkins made these and other assertions in a series of interviews here before yesterday's acknowledgement by South Africa that it has security forces in Zimbabwe-Rhodesia. Clearly embittered by his most recent experience there, Atkins, who earlier had been dismissed from the Prince George's County police force for misconduct, talked freely about conditions in the Rhodesian military and its prosecution of the war.

Ranking U.S. officials in a position to evaluate Atkin's assertions discounted some of them. They dismissed entirely his claim that South Africa is making contingency preparations to introduce nuclear weapons into the Rhodesian conflict should the black nationalist guerrillas appear on the verge of military victory.

However, these officials confirmed in detail the thrust of a number of Atkins' other allegations, particularly those reflecting a level of South African participation that would represent a major escalation of the war, from which the South African forces withdrew three years ago.

A spokesman for the South African Embassy yesterday described Atkins' charges as "absolutely absurd" but said that an unspecified number of South African forces is involved in protecting communication and rail links inside Zimbabwe-Rhodesia. The spokesman said Atkins' charges should be assessed in terms of his expulsion from Rhodesia.

An Israeli Embassy spokesman "very definitely" rejected the charges as untrue.

Atkins also said that Cubans, East Germans and at least two black Americans were among the recent casualties on the guerrillas' side in the conflict and that the Rhodesian forces depend heavily on white Americans, Britons, Australians and other foreigners.

U.S. officials confirm that South African troops and warplanes have taken part in the recent fighting, but they disputed Atkins' judgment that two combat battalions and a full squadron of Mirage fighter-bombers comprise the South African force in Zimbabwe-Rhodesia.

Atkins is no stranger to controversy. By his own account he was jailed, beaten and eventually expelled from Zimbabwe-Rhodesia because he was suspected of involvement in a coup plot hatched by right-wing whites. Atkins said he learned that such a plot did in fact exist, but he denied that he was involved in it.

During his nearly three-year stint in the Rhodesian forces, Atkins said he took part in more than 20 raids into neighboring Mozambique, Zambia and Angola.

One raid, he said, took him as far away as Tanzania, which supports Patriotic Front guerrillas but does not share a border with Zimbabwe-Rhodesia.

On many of the raids, Atkins said, small teams of paratroopers would free-fall from planes flying in commercial traffic lanes and carry out sabotage missions.

Atkins said South African Mirages took part in major battles in September in which Zimbabwe-Rhodesian forces attacked guerrilla strongholds in Mozambique. U.S. officials confirmed the use of Mirages in some of the fighting there, but said the warplanes could have been based in South Africa. Officially, Salisbury does not have any of the Frendh-designed planes but uses vintage British aircraft. Mozambique reportedly has up to 35 Soviet-built Mig21s flown by Cuban pilots.

Atkins said the South African Mirages were used to "soften up a target before we come in. They make a few strafing runs and orbit as long as they can in case anybody [from the guerrilla forces] is trying to come in. And that's it. They won't engage any heavy targets that are covered in armor and this type of thing."

He said the Mirages formerly were held at a restricted base in the Kruger National Park in northern South Africa, but have been moved to Buffalo Range in Zimbabwe-Rhodesia within the last few months.

In addition, Atkins said, the Rhodesians now have British-made Lynx helicopters supplied by South Africa during the April election campaign that brought Muzorewa to power. The helicopters, flown by South African pilots, were used to ferry various officials, candidates and journalists around the country and were left after the campaign ended.

Atkins said a small number of Israeli officers also were assisting the Salisbury government forces in an "advisory capacity" in Zimbabwe-Rhodesia and helping to train Rhodesian paratroopers at a base in Bloemfontaine in South Africa Transvaal region.

Atkins said he personally met three Isareli officers in South Africa and Zimbabwe-Rhodesia.

He estimated that 70 percent of Salisbury's paratroopers were trained in South Africa, mainly at Bloemfontaine, because Zimbabwe-Rhodesia's parachute school is too small to train the numbers of troops needed. He said the Israelis were there to "basically lend combat experience," which the South Africans lack.

Another Israeli connection Atkins cited was the delivery of 11 Huey helicopters originally bought by Israel from Italy, then sold to South Africa, which transferred them to Zimbabwe-Rhodesia. Italy makes Huey helicopters under license from the U.S. firm, Bell Helicopter, a subsidiary of Textron.

On the 11 helicopters, Atkins said, the Rhodesians have six left. Three crashed during training and two were shot down on a recent raid into Mozambique, he said.

No more Bell Huey helicopters are being bought, Atkins said, because the Rhodesians are short of foreign currency to pay for them.

Apart from denying the presence of any Israeli officers in South Africa and Rhodesia, Israeli diplomats also asserted that the 11 helicopters were sold by Israel to "an Asian country which sold the craft later to South Africa." It was earlier disclosed that the country involved was Singapore.

According to Atkins, the South Africans were "heavily involved" in Zimbabwe-Rhodesia when the first arrived in the country nearly three years ago but were later pulled out. South African troops and pilots wore their own uniforms and flew their own planes taking responsibility for certain areas of the country, he said.

Within the past several months, however, Atkins said, South African forces have moved back into Zimbabwe-Rhodesia -- this time wearing Rhodesian army uniforms and flying aircraft painted in the Rhodesian colors.

Atkins said he saw elements of two South African paratroop battalions based at Rutenga, south of Fort Victoria in the southeastern part of the country. He said the units have South African helicopters, armored personnel carriers and other material that make them much better equipped than the Rhodesian forces. He said the South Africans also have a tank force concentrated at Messina on Rhodesian border.

Atkins said the South African presence freed Rhodesian forces to concentrate on guerrillas operating from Zambia and northern Mozambique and to carry out raids in those countries.

Informed American officials confirmed that South African aircraft have been using the long airstrip at Buffalo Range. However, the officials said they doubted that as many planes as would comprise on or two squadrons are based there, but they conceded that there are "a lot of holes" in U.S. surveillance of the area.

U.S. intelligence reports confirm that some South African units have been operating in Zimbabwe-Rhodesia, notably at the Beitbridge border crossing between the two countries.

It was also in the Buffalo Range area that Atkins said he saw Rhodesian and South African pilots practicing an aerial maneuver normally used in delivering an atomic bomb from a fighter-bomber.

Atkins said he was familiar with the maneuver because he served with U.S.

NATO forces in Europe for two years and worked at a nuclear weapons facility near Heidelberg, West Germany.