NAIROBI, Kenya (AP) _ The governor of Rwanda's southern province of Butare was assassinated in an ambush, the prime minister said Sunday. He blamed soldiers of the defeated Hutu-led government living in neighboring Zaire.

Pierre-Claver Rwangabo was shot Saturday night near the town of Butare, the provincial capital near the border with Burundi, Prime Minister Faustin Twagiramungu said by telephone from Rwanda's capital, Kigali.

Rwangabo's son and driver also were killed, Twagiramungu said.

The prime minister said the killing of Rwangabo, a moderate Hutu, is believed to have been carried out by soldiers of the defeated Hutu-led government who have sought refuge in Zaire. An estimated 30,000 former soldiers and 10,000 allied militiamen are among some 1 million Rwandan refugees in the camps.

But a U.S. human rights group official said extremist Tutsis were more likely suspects. They had accused the governor of being too soft on Hutus accused in the massacres of a estimated 500,000 people, mostly Tutsis, from April to June last year.

After defeating the Hutu-led government last year, Tutsi-led rebels formed a government that includes moderate Hutus, such as Twagiramungu and Rwangabo.

Rwandan radio said the governor was returning home from Kigali when the assailants stopped his car by blocking the road with a tree. More than five gunmen in civilian clothes opened fire.

The broadcast, monitored by the British Broadcasting Corp., said his driver and son also were killed in the ambush, and Rwangabo's bodyguard, who returned fire, was seriously wounded.

``We believe he was killed by infiltrators from Zaire,'' Twagiramungu said, referring to former army soldiers and militiamen who fled to refugee camps in Zaire after being defeated by the Tutsi-led rebel Rwandan Patriotic Front.

But Allison Desforges of Human Rights Watch, who returned from Rwanda 10 days ago to her home in Buffalo, N.Y., said it seemed unlikely that refugees in Zaire would be able to penetrate into that region without being apprehended by the Rwandan army.

The region is highly populated and 40 to 50 miles from the Zaire border, she said.

Rwangabo, who supported freeing a priest suspected of genocide, was accused recently in the press of being too friendly to those who carried out the massacres, said Desforges.

``It's possible that people who held that opinion within Rwanda assassinated him,'' she said in a telephone interview. ``It's possible that it was an extremist Tutsi element.''

She said moderates were under attack from extremists on both sides.

Vice President and Defense Minister Paul Kagame, the commander of the rebel military victory, arrived in Butare on Sunday to lead investigations, the radio said.

``Let us remember that this despicable act comes at a time when there is talk of the arrest of ... elements of the former army who were involved in missions to destabilize the country,'' the broadcast quoted Kagame as said.

Rwandan troops have stepped up security to prevent incursions by Hutu extremists from refugee camps in eastern Zaire.