Eritrean forces shot dead hundreds of children and civilians in a November massacre in neighbouring Ethiopia's war-hit Tigray region, Human Rights Watch (HRW) said Friday.
It was the second major report on Eritrean abuses in the town of Axum, a UNESCO World Heritage site, in the past week.
An Amnesty International investigation into the same events detailed how Eritrean troops "went on a rampage and systematically killed hundreds of civilians in cold blood."
The findings from the rights watchdogs come as global concern mounts over atrocities by Eritrean troops in Tigray.
UN leaders on Thursday accused the Eritreans of possible crimes against humanity and urged them to pull out.
Addis Ababa and Asmara deny Eritrea is actively involved in Tigray.
Prime Minister Abiy Ahmed announced military operations against the leadership of the Tigray People's Liberation Front (TPLF), Tigray's then-ruling party, in early November, saying they came in response to TPLF attacks on federal army camps.
Ethiopian and Eritrean forces entered Axum on November 20 after "indiscriminate" shelling that killed civilians, said the HRW report published Friday.
The Eritreans then engaged in "widespread pillaging" as Ethiopian troops mostly looked on, the report said.
"I asked one soldier, why are you not doing anything, you are Ethiopian, and we are in Ethiopia; you are allowing the Eritreans to do this," it quoted one resident as saying.
"He told me: We need an order from above."
The massacre began on November 28 after Tigrayan militia members, joined by some residents, attacked Eritrean soldiers, HRW said.
After calling in reinforcements, the Eritreans began "moving through the town, going house to house, searching for young men and boys, and executing them."
- 'Curtain of denial' -
Like Amnesty, HRW said it was impossible to provide an exact death toll but estimated that "over 200 civilians were most likely killed on November 28-29 alone."
That would make the Axum massacre one of the deadliest atrocities of the conflict so far.
Last week AFP travelled to the Tigray village of Dengolat to document a separate massacre by Eritrean troops at around the same time that church officials said left 164 civilians dead.
Since the publication of Amnesty's report, Abiy's government has said federal investigators are probing "credible allegations" of atrocities and abuses including in Axum.
But the government has also tried to cast doubt on Amnesty's findings and accused it of "reinforcing the misinformation and propaganda by TPLF and its cohorts."
HRW called Friday for an urgent UN investigation into possible war crimes and crimes against humanity in Tigray.
"Eritrean troops committed heinous killings in Axum with wanton disregard for civilian lives," HRW Horn of Africa director Laetitia Bader said.
"Ethiopian and Eritrean officials can no longer hide behind a curtain of denial, but should allow space for justice and redress, not add to the layers of trauma that survivors already face."