JOHANNESBURG, OCT. 17 -- South African police announced today that they had arrested three white men in connection with an ambush of a bus outside Durban last week that killed six and wounded all the other 27 black passengers.

The arrest of the whites is likely to fuel already widespread speculation that some kind of "third force" has been acting behind the scenes to stir up violence in the black community in efforts to undermine the fledgling negotiating process between the government and African National Congress.

Police here also have made arrests in another recent terrorist incident. Five blacks, described in police reports as residents of a predominately Zulu hostel near Johannesburg, have been charged with murder and attempted murder in connection with a Sept. 13 attack aboard a Soweto-bound commuter train that killed 26 blacks and injured more than 100.

That incident alarmed the government, leading President Frederik W. de Klerk to speak of "a new dimension" to the black political violence that took more than 750 lives in August and September. A few days later, he ordered a major security crackdown in the townships. Fighting between rival black factions soon stopped, as did terrorist incidents.

Now, however, there has been a new outbreak of terrorist attacks in Durban. Sources interviewed there earlier this week said a white extremist group had taken responsibility for the Oct. 9 bus attack, claiming it was in retaliation for the stabbing of nine whites by a gang of blacks who went on a rampage in downtown Durban the same day.

The authorities have not publicized that a white extremist group claimed responsibility for ambushing the bus because of fear that blacks might seek revenge against whites, the sources said.

A police statement issued in Durban today gave few details about the three whites except to say they had been arrested in the past 24 hours as the result of an intensive investigation into the bus incident.

A Law and Order Ministry spokesman said later, however, that the three men belonged to the neo-Nazi Afrikaner Resistance Movement. The South African state radio said they had been arrested in Richard's Bay, about 100 miles north of Durban.

The nighttime attack on the bus took place along a three-lane highway that connects Durban to the black township of Kwamashu, 12 miles north of Durban. An unknown number of men opened fire with automatic rifles from a car as it passed the bus, leaving all 33 black passengers dead or wounded.

The same night, a Durban newspaper received a call saying the terrorist action was the work of white extremists avenging an attack by blacks on whites near the Durban beachfront that morning and warning that similar retaliatory action would be taken if more whites were hurt.

Nine whites were stabbed by a gang of 12 to 20 blacks who allegedly stole knives from a shop near the downtown beachfront and then attacked whites indiscriminately. One of the whites, an 85-year-old man, died Monday from his wounds. A 17-year-old male remains in critical condition.