Gunmen in western Ethiopia killed at least 34 people in an attack on a bus on Saturday night, the country's national human rights body said on Sunday.

The Ethiopian Human Rights Commission said the number of people killed was likely to rise after what it called a "gruesome" attack on a passenger bus in the Benishangul-Gumuz region.

"The latest attack is a grim addition to the human cost which we bear collectively," Daniel Bekele, the chief commissioner, said in a statement.

The violence comes amid a 12-day-old war between the Ethiopian government and the restive Tigray region in the country's north.