Black-on-Black Strife Spreading in S. Africa : Infighting: At least 18 are killed in Transvaal. It is the most serious trouble outside Natal since feuding began.

The bloody internecine strife that has ravaged Natal province, claiming 3,000 lives in three years, has spread to a black township in Transvaal province, where at least 18 people were stabbed or bludgeoned to death over the weekend, police said Monday.

The deaths in Sebokeng, about 30 miles south of Johannesburg, marked the most serious outbreak of Zulu violence outside Natal since the feuding began between supporters of the African National Congress and Zulu chief Mangosuthu Gatsha Buthelezi.

It came only a week after Buthelezi announced a nationwide membership drive for his new Inkatha Freedom Party, and it suggested a dangerous and widening split between Inkatha and the ANC as the two political forces attempt to gain strength among the country’s 27 million blacks.

Meanwhile, two weeks before the next round of ANC-government peace talks, tensions grew Monday between the ANC and the government over the recent arrest of at least 40 ANC guerrillas and the seizure of weapons.

President Frederik W. de Klerk on Monday night warned the ANC that “disorder, anarchy and terrorism will not be tolerated.” And he emphasized that the temporary immunity granted ANC leaders for the peace talks “applies only (to) actions before the . . . temporary immunity came into operation. The peace and immunity processes do not provide room for anybody to sneak in through the back door and try to seize power in the country by force.”

Two South African newspapers quoted unnamed security sources Sunday as suggesting that the arrests were linked to a movement inside the ANC to overthrow the government if it didn’t get its way in negotiations.

ANC internal leader Walter Sisulu said the ANC is committed to negotiations, but he pointed out that the ANC has not yet agreed to a cease-fire. He added that the detained operatives had been performing normal activities. The ANC has said it will consider a cease-fire when all obstacles to negotiations, including the release of political prisoners, have been removed.

The trouble in Sebokeng began Sunday afternoon when about 1,000 Inkatha supporters, some carrying spears and other traditional weapons, showed up for a political rally. Inkatha does not have substantial support in the township, and many of the participants were brought to the rally by bus from Natal.

Several hundred ANC supporters appeared outside the rally and began throwing rocks at police cars, the authorities said. Later, fighting broke out at a dormitory housing 20,000 men, most of them mine workers. A police officer was killed by a Zulu spear thrown from a crowd, and the authorities later recovered 17 bodies at the dormitory.

“It is a political strategy of Inkatha to spread violence beyond the boundaries of Natal,” said Jay Naidoo, the secretary general of the Congress of South African Trade Unions, an ally of the ANC.

Buthelezi countered that Inkatha is being attacked by the ANC and others “who do not want us to be part of a future democracy.”