A top official of South Africa's Bureau of State Security has been in Kinshasa secretly to negotiate emergency aid - mainly fuel and support funds - to help Zaire hold off Katangan rebels in southern Shaba Province, according to three well-placed sources here.

The high-ranking official of BOSS - the powerful South African equivalent to the CIA and FBI combined - was reportedly here last week to talk with authorities in the office of President Mobutu Sese Seko.

The highly reliable sources said the Zaire government accepted the offer of an undisclosed amount of fuel - of which there is a chronic shortage here - and funds, perhaps credit to buy South African goods.

The South African official apparently did not discuss arms or military advisers, a sensitive area in light of South Africa's disastrous involvement in the three-way Angolan civil war in late 1975 and early 1976. The government of Prime Minister John Vorster received international condemnation for that involvement.

The disclosure is not a surprise in light of the strong behind-the-scenes relations between Zaire and South Africa, which have been secured recently because of mutual political, economic and military concerns.

Other well-informed sources disclosed today that a Moroccan contingent that is coming to aid the Zaire army will include 1,500 paratroopers led by two colonels who served with the United Nations forces in Zaire in the early 1960s.

The Moroccan force, originally expected here today, is now scheduled to arrive early Saturday.

Several Moroccan supply planes are expected over the next few days with support equipment for Kolwezi, the strategic mining center of Shaba Province believed to be the next target of Katangese rebels.

Western sources here explained today that Morocco's King Hassan had decided to commit his troops because of his belief that Cuba is behind the dispute over the Spanish Sahara.

Zaire, which has charged that the Cubans, Soviets and Angolans are behind the Katangan "invasion," cut off relations with Cuba this week charging that a Cuban diplomat in Kinshasa had been found with "irrefutable evidence" of Cuba's involvement in the month-old rebellion.

Hassan, after discussions with Zaire Foreign Minister Nguza Karl-I-Bond, apparently decided that Zaire's allies should help block further Cuban aggression.

The Zairian foreign minister met with Hassan in Rabat on March 25 and told Radio Rabat then that he had given Hassan a "detailed explanation of the deteriorating situation in Zaire." He said he had "found great understanding from King Hassan to my mission" and was bringing to Mobutu a reply - presumably the pledge to send troops.

At a press conference at the Morcoccan embassy here today, the ambassador said Morocco is operating within the framework of the United Nations and Organization of African Unity charters, which condemn aggression by external forces across national borders.

The South African and Moroccan response to Zaire's appeals for aid reflect the seriousness with which some African countries view the Katangan rebellion.

South Africa fears the loss of the Mobutu government, one of its strongest allies in black Africa - a relationship that has grown over the past three years.

Zaire buys foods and medicine from South Africa. In Kinshasa supermarkets, cans of silver-leaf peas processed in Johannesburg can be found next to peas imported from China. About 60 per cent of the corn in Shaba Province reportedly comes from South Africa.

Local housewives flock to the butchers when word gets out that a South African beef shipment has arrived. When Henry Kissinger was here last year, Mobutu served him South African meat.

Local economists claim that about 90 per cent of the mining equipment used in Shaba comes from South Africa. Much of the copper ore from Shaba is routed, via Zambia and Rhodesia, to South Africa's Port Elizabeth.

That route has become increasingly important for Zaire since the Benguela Railway, one of Africa's most important transportation links, has been closed since 1975 by the civil war in Angola through which it passes.

Angola also helped bring Zaire and South Africa closer politically in their shared support of the pro-Western movements against the Cuban-backed Marxist faction that won the three-way civil war.

A small contingent of South African troops fought in the north alongside the National Front, having entered Angola through Zaire.

Well-placed sources said that the two governments conferred frequently then and maintained constant contact during South Africa's presence in Angola.

On another political level, Zaire's President Mobutu has traditionally been one of the most moderate African leaders on the question of a settlement in Rhodesia.

If a Marxist government came to power in Rhodesia, the pro-Western Zaire government fears loss of access to the Rhodesian coal that is essential to Shaba's mining operation.