An earlier Amnesty International investigation into the same events described how Eritrean troops "went on a rampage and systematically killed hundreds of civilians in cold blood."
The findings from the two rights organisations come as global concern mounts over atrocities by Eritrean troops in Tigray.
The U.N. Security Council met behind closed doors Thursday to discuss the ongoing conflict but issued no statement afterwards.
UN leaders on Thursday accused the Eritreans of possible crimes against humanity and urged them to withdraw.
Addis Ababa and Asmara deny Eritrea is actively involved in Tigray.
Ethiopian troops stood by, doing nothing
Ethiopian 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 the military action was in response to TPLF attacks on federal army camps.
Held a review meeting with the Tigray Region SOE Taskforce and the Provisional Administration to discuss activities and progress in the region. As of today, the SOE Task Force will continue its stabilisation & rebuilding efforts under the leadership of Gen. Yohannes Gebremeskel. pic.twitter.com/ndpYSdSLwM
— Abiy Ahmed Ali 🇪🇹 (@AbiyAhmedAli) March 3, 2021
Ethiopian and Eritrean forces entered the town of Axum on 20 November after "indiscriminate" shelling that killed civilians, according to the Human Rights Watch (HRW) report published on Friday.
The Eritreans then engaged in "widespread pillaging" as Ethiopian troops mostly looked on, the report said.
The massacre began on 28 November after Tigrayan militia members, joined by some residents, attacked Eritrean soldiers, HRW said.
Eritrean armed forces massacred scores of civilians, including children as young as 13, in the historic town of Axum in Ethiopia’s #Tigray region in November 2020 - Human Rights Watch @hrw https://t.co/bsQtaL5jSA
— Ashok Swain (@ashoswai) March 5, 2021
After calling in reinforcements, the Eritreans began "moving through the town, going house to house, searching for young men and boys, and executing them".
Call for urgent UN investigation
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".
Last week the French AFP news agency travelled to the Tigray village of Dengolat to document a separate massacre by Eritrean troops at around the same time as the Axum masscre. Church officials say the Dengolat attack left 164 civilians dead.
HRW called Friday for an urgent UN investigation into possible war crimes and crimes against humanity in Tigray.
The United Nations high commissioner for human rights says a request has already been put to Ethiopia to permit an investigation into allegations of murders and sexual violence in the Tigray region, which may amount to war crimes.
“We urge the government of Ethiopia to grant my office and other independent monitors access to the Tigray region, with a view to establishing the facts and contributing to accountability, regardless of the affiliation of the perpetrators,” Michelle Bachelet said Thursday in a statement.
Thousands of people have been killed in the area where hundreds of thousands of people were dependent on food aid before the conflict began, according to the U.N.
