The UN human rights chief has called for an inquiry into reports that up to hundreds of people had been stabbed and hacked to death in one town.
Prime Minister Abiy Ahmed has accused fighters from the Tigray People's Liberation Front of the massacre.
Tigrayan officials have denied involvement in the atrocities.
Mr Abiy said that TPLF fighters went on the rampage after federal troops had "liberated" the western part of Tigray, "brutally" killing innocent civilians in Mai-Kadra, a town in the South West Zone of Tigray.
Witnesses blamed forces loyal to TPLF for Monday's killings - first reported by human rights group Amnesty International.
If confirmed, this would be the first large-scale killing of civilians in the fighting between government forces and the TPLF which broke out last week.
Getting information about the clashes is hard because phone lines and internet services are down.
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