US authorities have taken issue with Turkey over its sales of armed drones to Ethiopia, where two sources familiar with the matter said there was mounting evidence the government had used the weapons against rebel fighters.

Washington has “profound humanitarian concerns” over the sales, which could contravene U.S. restrictions on arms to Addis Ababa, a senior Western official said.

The year-long war between Ethiopia’s government and the leadership of the northern Tigray region, among Africa’s bloodiest conflicts, has killed thousands of civilians and displaced millions.

A State Department spokesman said U.S. Horn of Africa envoy Jeffrey Feltman “raised reports of armed drone use in Ethiopia and the attendant risk of civilian harm” during a visit to Turkey last week.

A senior Turkish official said Washington conveyed its discomfort at a few meetings, while Ethiopia’s military and government did not respond to detailed requests for comment.

Turkey, which is selling drones to several countries in Europe, Africa and Asia, has dismissed criticism that it plays a destabilising role in Africa and has said it is in touch with all sides in Ethiopia to urge negotiations.

Last week the United Nations agreed to set up an independent investigation into rights abuses in Ethiopia, a move strongly opposed by its government.

Read more: Reuters