Sudan's military has accused the Ethiopian army of executing seven Sudanese soldiers and a civilian who were captives.
A statement published on Sudanese state media gave no further details.
But it called the alleged killings a treacherous act, and said there would be a response.
There has been no comment so far from the Ethiopian side.
A long-running border dispute has flared up over the last two years, with occasional clashes in the al-Fashaga area.
Sudan has also been angered by Ethiopia's construction of a giant dam on the River Nile.