Foreign Minister Gedion Timotheos accuses Eritrea of collaborating with rebel groups in northern Ethiopia.
"Developments in recent days indicate that the Eritrean government has chosen continued escalation," he writes in a letter to his Eritrean counterpart.
When massive fighting raged in the Tigray region almost four years ago, Eritrea intervened with forces on the side of the Ethiopian government. The Ethiopian government only admitted this earlier this week, at a time when relations between the two countries had already been deteriorating.
