Six Hutu rebels have been charged in the 1997 killings of four U.N. human rights monitors, state-run Rwandan radio reported Saturday.

The monitors and their driver were ambushed near Cyangugu, 150 miles southwest of Kigali and on the border with Congo, where the rebels pass easily in and out of the country.U.N. human rights spokesman Jose-Luis Herrero said the six accused were captured in Cyangugu in February 1997. He had no other details.

The Spanish Foreign Ministry, meanwhile, said that two Spanish nuns kidnapped by Hutu rebels six days ago were freed by their captors Saturday. It was not clear how or why they won their freedom.

The six Hutu rebels charged with involvement in the 1997 killings of U.N. monitors also had been charged with armed robbery and state security.