MANAGUA, NICARAGUA, OCT. 20 -- Rebels kidnaped an American working with the Witness for Peace group in central Nicaragua, a spokesman for the organization said today.

Paul Alan Fisher was abducted Saturday near La Libertad in Chontales province, 103 miles southeast of here, according to the group's local director, Ed Griffin Nolan.

In Washington, Sam Hope, a spokesman for Witness for Peace, said Fisher's hometown is Mill Valley, Calif.

In Miami, the Nicaraguan Resistance, the rebels' umbrella organization, said today they knew nothing of the report.

Nolan said Fisher was part of a 32-member team from the organization that went around the Nicaraguan countryside checking on human rights violations. Nolan said the last time the group heard from Fisher was on Friday when he said he was going to Santo Domingo, about 12 miles east of La Libertad.

The Sandinista newspaper Barricada said Sunday that rebels had ambushed two Ministry of Mines vehicles Saturday about five miles south of La Libertad, killing three persons and kidnaping four others.

"The kidnaped people were not identified" in the newspaper, Nolan said, "but we are sure it was Fisher."

Nolan said Fisher was among those who contributed to a recently published report by the peace organization entitled "Civilian Victims of the U.S. Contra War." The report said 350 people have been victims of ambushes, kidnapings, attacks, murders and explosions caused by the rebels from January through June.

Witness for Peace, which opposes U.S. aid to the rebels, sponsors visits to Nicaragua by Americans who gather information on the conduct of the war.

Meanwhile, former rebel leader Edgar Chamorro returned to Nicaragua today under a government amnesty. He said he left the rebel movement because he did not agree "with the injustices and atrocities that the forces backed by the United States commit.