The United States believes Eritrean soldiers have crossed into Ethiopia's Tigray region, according to a U.S. government source and five regional diplomats, despite denials from both countries.
The two nations, led by Prime Minister Abiy Ahmed and President Isaias Afwerki, made peace in 2018 and now have a mutual foe in the Tigrayan People's Liberation Front.
Though Ethiopia's government says a month-long offensive against the TPLF is now over, a spokesman on Tuesday (December 8) said some fighters in the northern region were not yet defeated.
Eritrea has, however, repeatedly denied sending its soldiers to support the Ethiopian military.
The U.S. view of the conflict appears to disagree.
According to the Reuters sources, who were all briefed on the U.S. assessment, evidence of Eritrean involvement includes satellite imagery, intercepted communications and anecdotal reports from the ground.
A U.S. government source said there did not appear to be "any doubt" while a senior diplomat from another country said "thousands" of Eritrean soldiers were believed to be engaged.
The U.S. State Department did not confirm the U.S. conclusion, but said its embassy in the Eritrean capital Asmara was urging restraint to officials.
Abiy's spokeswoman said queries should be directed to Eritrea, whose foreign minister called suggestions of Eritrean involvement "propaganda."
The U.S. assessment does match reported sightings by Tigrayan residents and refugees, however, and the TPLF claims to have killed and captured large numbers of Eritrean soldiers over the past month.
With communications down and the access tightly controlled, claims from Tigray are near impossible to verify but reports of Eritrean involvement will alarm the U.S. and other world powers.
Ethiopia hosts the African Union, its security services work with Western allies and its troops serve in peacekeeping missions.
Eritrea, however, has for years faced accusations of large scale rights abuses, including jailing opponents and forcing citizens into lengthy military or government service.