Amnesty International said civilians are being slaughtered in Ethiopia's Tigray region. The United Nations added it was impossible to get to the area.
Amnesty says an escalating armed conflict between the country's military and regional groups has resulted in a worsening humanitarian crisis.
Claims of massacres have been reported since the start of the week. News organizations haven't confirmed the number of deaths but most say a "very large number" of civilians were victims.
"We have confirmed the massacre of a very large number of civilians who appear to have been day laborers in no way involved in the ongoing military offensive," Amnesty said.
Journalists and human rights groups have not been able to get information about the conflict because communications in the Tigray region remain down.
Following the escalation of raids and attacks on civilian areas and government infrastructure, Ethiopia's federal government declared "war" with the Tigray People's Liberation Front and other regional groups. Ethiopian Prime Minister and Nobel laureate Abiy Ahmed said he was unwilling to negotiate and fully intended to disarm and arrest their leaders.
Ethiopia's army has conducted "law enforcement operations" in the region and have bombed known encampments with airstrikes.
Earlier in the week, state-affiliated news media reported federal forces had killed more than 550 rebels. The identities and affiliations of those that were killed weren't disclosed.
Witnesses in the town of Mai-Kadra said they saw many bodies in the streets. Amnesty said most of those that came to them were wounded. Witnesses said injuries were inflicted by axes, knives and machetes.
More than 10,000 people have fled to neighboring Sudan. The U.N.'s High Commissioner for Refugees estimates that half the refugees were children.
The U.N.'s Office for the Coordination of Humanitarian Affairs says there might be 600,000 people facing starvation, 100,000 internally displaced and 96,000 refugees.