Reports of the mass killing of civilians in Ethiopia's northern Tigray region "would amount to war crimes" if confirmed, the UN has warned.
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.
Ms Bachelet said she was "alarmed" about the situation in Ethiopia, but added that the "first priority" was to stop the fighting and prevent further atrocities.
There has been long-standing tension between Ethiopia's government and the TPLF, which controls Tigray, the country's northernmost state. The tension has boiled over into military clashes, including air strikes by federal forces.
As a result, thousands of civilians have crossed the border into Sudan, which says it will shelter them in a refugee camp.
Who were the victims?
Amnesty said the killings happened on the night of 9 November.
The human rights group said it had confirmed that "scores, and likely hundreds, of people were stabbed or hacked to death in Mai-Kadra (May Cadera) town".
It said it had seen and "digitally verified gruesome photographs and videos of bodies strewn across the town or being carried away on stretchers".
Amnesty said the victims appeared to be labourers not involved in the conflict. It is not clear where they came from.
It said witnesses had spoken of wounds "inflicted by sharp weapons such as knives and machetes". Some witnesses said the attacks were carried out by forces loyal to the TPLF after they had been defeated by federal troops in an area called Lugdi.
