The U.S. called for the immediate withdrawal of Eritrean troops from neighboring Ethiopia’s Tigray region, following “credible” reports of their involvement in a civil conflict.

The reports relate to an assault that Ethiopian Prime Minister Abiy Ahmed ordered on the northern regional state Nov. 4, after blaming it for an attack on a military base to try and steal weapons. Eritrea’s involvement would confirm that the conflict has become regional.

“This is a grave development,” a U.S. State Department spokesperson said in an emailed statement.

The U.S. adds credence to accusations by the former rulers of Tigray that Eritrea is supporting the Ethiopian army against them. United Nations officials said earlier they’ve observed troops wearing Eritrean uniforms in the region.

Abiy has stressed that the conflict in Tigray is an “internal affair” and a “law enforcement operation.” At the onset of the fighting, Abiy accused the Tigray People’s Liberation Front of manufacturing uniforms resembling those of Eritrea’s army to “implicate the Eritrean government in false claims of aggression against the people of Tigray.” Abiy’s spokesperson didn’t immediately respond to a message requesting comment on Friday.

Eritrea hasn’t commented on whether its forces are involved in the conflict. The nation’s information minister didn’t immediately respond to a message seeking comment.

The U.S. said it’s also aware of reports of human rights violations in the region, and called for an independent investigation of the matter.

“All parties must respect human rights and international humanitarian law,” the Sate Department said.