Gunmen in western Ethiopia have killed at least 34 people in an attack on a bus, the national human rights body said Sunday.

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

Details of the attack remained unclear in a country that has seen escalating violence in recent weeks.

The EHRC also said that "there are reports of similar attacks" in three other areas, including on people "who have fled to seek shelter."

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

Read more: Deutsche Welle