Prime Minister Pieter W. Botha today acknowledged that "for some time now" South African security forces have been operating inside Zimbabwe-Rhodesia to protect South African "interests" such as the railway line.

Botha's statement is the first official confirmation of a South African military presence in Zimbabwe-Rhodesia since Pretoria said it had withdrawn the last of its police forces in 1976.

"South Africa, after consultation with the government of Zimbabwe-Rhodesia, has for some time now been looking to the protection of our interests as well as our vital lines of communication such as the rail link to Beitbridge and the railway links through it," Botha said in Pretoria. He was speaking at a farewell function for the retiring chief of the air force.

While the prime minister did not specifically mention South African troops, his remarks strongly implied this and defense officials did not deny that the "protection" he spoke about was being carried out by soldiers.

A defense spokesman refused to elaborate on how long the "protection" had been going on or what it entailed, but it is well-known that South African troops have been stationed on both sides of the rail and road border post of South Africa and Zimbabwe-Rhodesia at Beit Bridge for more than two years.

While seeking to define their presence as one protective of its interests, the South Africans, by the official acknowledgement, could place a major stumbling block at the London conference on Rhodesia now centering on arrangements for a cease-fire.

[Despite earlier optimism that a settlement was in sight, the conference remained deadlocked yesterday after leaders of the Patriotic Front presented Lord Carrington with new cease-fire plans differing significantly from the British proposals, Washington Post special correspondent James Lemoyne reported from London].

South African officials refused to say how many troops the country has in Zimbabwe-Rhodesia. According to unconfirmed but reliable reports in Salisbury, there are as many as a battallion and possibly two, who operate under their own command headquarters at Fort Victoria in southern Zimbabwe-Rhodesia.

Presumably the protection exercise entails patrols of the railway line from Beitbridge which branches into two lines, one going to Salisbury and the other to Victoria Falls and then into Zambia. This line carries imports and exports of Zambia as well as goods of Zimbabwe-Rhodesia and this is the rationale Botha invoked for protecting it.

"It is important that our trade routes to Zimbabwe-Rhodesia and states further north shall be protected in the interests of all parties and inhabitants of our subcontinent," Botha said.

South Africa's military cooperation with the Zimbabwe-Rhodesian government forces would appear to be considerably more, however, than simply protecting its railroads. South African military officers are allowed to serve in the Zimbabwe-Rhodesian forces for extended periods without losing their seniority or rank when they return to the South African defense force.

South African helicopter pilots are reported to be flying for the Salisbury government. In early September, three South African air force members went down during a raid by Zimbabwe-Rhodesian forces into neighboring Mozambique, according to several reliable sources. The defense force refused to disclose where the men died.

In addition, infantrymen have been asked to volunteer for brief duties with Zimbabwe-Rhodesian troops in southern part of the country.

Botha today said that anti-South African guerrillas were "collaborating with the Patriotic Front," implying that this was another reason why his troops were in Zimbabwe-Rhodesia. However, up to now, South African authorities have never publicly complained about infiltration of insurgents from Zimbabwe-Rhodesia. Most of the South African guerrillas, who belong to the African National Congress, come into South Africa from Mozambique and Botswana.

Today's announcement appears to have been prompted by reports in the British Press of active South African military involvement in the conflict between the white-led government forces of Prime Minister Abel Muzorewa and the Soviet-backed guerrilla alliance, the Patriotic Front.

By admitting a limited South African military presence with a specific mission, Botha's statement apparently is intended to put the South African military presence in Zimbabwe-Rhodesia "in perspective" and should be regarded as a "denial" of its involvement in the conflict itself, according to an official South African source who did not wish to be named.

In Washington, a State Department official quoted by Reuter said President Carter intends to lift trade sanctions against Zimbabwe-Rhodesia promptly after a British governor takesover in Salisbury, the capital.

[Richard Moose, assistant secretary of state for African affairs, made the pledge to the Senate Foreign Relations Committee which is trying to head off a move in Congress to compel the president to lift sanctions immediately.]