At least 45 civilians were killed in door-to-door raids by Ethiopian troops last month in the northern town of Merawi, the country’s human rights watchdog says.
Witnesses told the BBC a pregnant woman was among those shot.
She later died in hospital, according to medics.
The Ethiopian government has not commented on the killings. Both the US and EU are calling for an independent investigation.
It is one of the worst episodes of violence in the Amhara region since last August, when powerful local Fano militias began a rebellion against the government’s plans to disarm them.
Before then, Fano had been an ally of Ethiopian troops fighting their common enemy – the TPLF rebels in the neighbouring Tigray region.
A peace deal has now been agreed in Tigray.
Fano do not want their forces to be disbanded because they fear it would leave them exposed to attacks from neighbouring regions.
Merawi residents describe several hours of fierce fighting on 29 January between the army and Fano fighters, followed by house-to-house searches by uniformed Ethiopian security officers.
‘I saw bodies lying on the roads’
Numerous witnesses spoke to the BBC last month on condition of anonymity.
“They went into my brother’s house… They brought him and 12 others out to the streets and shot them,” a labourer told the BBC.
“The soldiers were threatening us [and] accusing us of sheltering the Fano and providing them food,” said another resident, who is adamant the civilian massacre was revenge for militia attacks on government troops.
Another eyewitness is grieving her younger brother, who was a civil servant.
“He was with his son. They told me to hold the child. They took [my brother] and killed him.”
Most of those killed were young men, according to witnesses.
One of the most disturbing testimonies came from medical staff, who confirmed to the BBC that “a pregnant woman was among those shot. She died after arriving at the hospital.”
A man who managed to hide during the raids on Merawi town and then run away described the aftermath.
“When I got out the next day, I saw bodies lying on the roads.”
Details have only emerged in recent days because a months-long internet blackout covering most of Amhara has hampered communication.
A total of 45 dead who died at the hands of government troops have been identified, the Ethiopian Human Rights Commission (EHRC) said on Tuesday, “however, it can be assumed that the number of victims is even higher”.
Ethiopian MPs last week voted to extend the state of emergency currently in place in Amhara – the country’s second-most populous region – by a further four months.
Both the US and the European Union have expressed their concerns at the move, which comes amid intense ongoing fighting between Fano militias and the army.
At its height, local Amhara militias briefly controlled a major airport and overran most of the major cities in the region.
The military has since regained control of the largest cities but fighting continues to be reported in rural villages and smaller towns like Merawi.
Drone strikes and heavy artillery have been used in Amhara, where the violence has created a widespread sense of anxiety.
A civil servant in Bure, another small town that saw frequent clashes in recent weeks, told the BBC that government offices have not been fully operating for months because of the conflict.
“Heavy weapons are fired randomly and sometimes [the violence] goes on for hours.”
According to the UN, civilian targets including a school compound and a bus station were hit by drones in December.
At the time, Ethiopia’s central government denied targeting civilians but vowed to use all tools at its disposal to eradicate who it called “extremists”.