Sudan's foreign ministry said on Monday it "will immediately recall its ambassador to Ethiopia for consultations" and submit a complaint with the UN Security Council.
"In an act that contravenes all laws and customs of war and international humanitarian law, the Ethiopian army executed seven Sudanese soldiers and a citizen who were their captives," the Sudanese armed forces said late Sunday.
A Sudanese military official who requested anonymity told AFP the soldiers were taken into captivity from a border area close to the Al-Fashaqa region.
The army said "this treacherous act will not pass", vowing to respond to "this cowardly behavior."
"The Ethiopian ambassador to Khartoum will also be summoned to inform him of Sudan's condemnation of this inhumane behavior," the ministry said.
Relations between Khartoum and Addis Ababa have soured over Al-Fashaqa, a fertile strip long cultivated by Ethiopian farmers but claimed by Sudan, sparking sporadic deadly clashes between the Sudanese and Ethiopian sides.
Tensions were heightened further after fighting erupted in Tigray in November 2020, sending tens of thousands of refugees fleeing into Sudan.
The border dispute feeds into wider tensions in the region, including over Ethiopia's controversial Blue Nile dam.
Sudan and Egypt, both downstream countries, have been opposed to the Great Ethiopian Renaissance Dam and pushed for an agreement over the filling of its reservoir and the operation of the dam.
In February, Khartoum and Cairo slammed Addis Ababa for unilaterally deciding to start power generation at the dam.