The United States believes reports of Eritrean military involvement in the conflict in Ethiopia’s Tigray region are “credible,” a State Department spokesperson told Reuters on Thursday, despite denials by both nations. The spokesperson called on any Eritrean soldiers there to pull out. “We are aware of credible reports of Eritrean military involvement in Tigray and view this as a grave development. We urge that any such troops be withdrawn immediately,” the spokesperson said. Reuters was first to report on Tuesday that the U.S. government believed Eritrean soldiers had crossed into Ethiopian territory, effectively helping Ethiopian Prime Minister Abiy Ahmed’s government battle a rebellious northern force. Abiy won a Nobel Peace Prize last year for making peace with Eritrea, but the presence of Eritrean troops on Ethiopian soil would alarm Western allies and risk further inflaming the conflict. Eritrea has for years faced accusations of large scale rights abuses, including jailing opponents and forcing citizens into lengthy military or government service. It accuses Western powers of smear campaigns and luring Eritreans abroad, which they deny.