JOHANNESBURG, JULY 18 -- African National Congress leader Nelson Mandela returned tonight from a six-week, 14-nation trip clearly buoyed by the international support he had received for his black nationalist organization's position on continuing sanctions against Pretoria and saying he was "very keen" to resume talks with the South African government.

At an airport news conference here, Mandela said he plans to see President Frederik W. de Klerk "in the next few days" to set a date for the next meeting between the ANC and the white-minority government to resolve the remaining obstacles to formal negotiations for a new national constitution.

The ANC leader -- who marked his 72nd birthday today, his first outside jail in 28 years -- denied that the ANC had been dragging its feet on the "talks about talks." These concern the release of hundreds of political prisoners and the return of 20,000 anti-apartheid exiles, which the ANC regards as the two main remaining obstacles to formal negotiations.

Mandela said that as soon as Pretoria resolves these issues, the ANC would consider the "suspension of hostilities" against the government, but "not the end of the armed struggle" or even a cease-fire declaration.

Mandela, who is recovering from a slight case of pneumonia, answered questions tersely and told a small crowd of supporters at the airport later that he had been delayed several hours because of a bomb threat to his plane in Maputo, the capital of Mozambique. There also was a bomb threat at the airport here, causing police to clear the room where the news conference took place and to bring in dogs to check for explosives.

Meanwhile, at least 26 persons died near Durban in Natal province today when unidentified gunmen opened fire on a bus in the latest outbreak of violence between Zulu supporters of the ANC and those loyal to a rival Zulu tribal organization. Sixty-five people were injured in the bus and a van caught in the ambush. It was by far the highest number of casualties in any one day of bloodshed involving the two groups.