U.S. officials believe reports of Eritrean military forces in the Tigray region of Ethiopia to be “credible,” a State Department spokesperson told Reuters.

Reuters first reported Tuesday that a U.S. government source and five regional diplomats had been briefed that Eritrean soldiers were helping Ethiopian Prime Minister Abiy Ahmed’s government fight against the region’s opposition group Tigray People’s Liberation Front (TPLF).

However, both Ethiopia and Eritrea denied the presence of Eritrean forces in the region.

“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 We urge that any such troops be withdrawn immediately,” the spokesperson told the news service.