Ethiopian and Eritrean forces committed war crimes during an offensive to take control of the town of Axum in Ethiopia’s northern Tigray region late last year, with one massacre by Eritrean troops a potential crime against humanity, according to a report released by Amnesty International on Thursday.

The human rights group said that hundreds were likely killed during a roughly 24-hour period over Nov. 28-29, when Eritrean soldiers carried out house-to-house searches and shot civilians on the street.

Eritrean troops “went on a rampage and systematically killed hundreds of civilians in cold blood, which appears to constitute crimes against humanity,” said Deprose Muchena, Amnesty International’s director for east and southern Africa.

The United Nations defines crimes against humanity as “widespread or systematic attack directed against any civilian population.”

In its report, Amnesty calls for a U.N.-led investigation into the violence in Axum as part of a broader international inquiry of the conflict between the Ethiopian government and forces aligned with the Tigray People’s Liberation Front (TPLF) that began on Nov. 4.

The report comes amid mounting international pressure on Ethiopian Prime Minister Abiy Ahmed, who came to power in 2018, to restore peace. Ethiopia receives hundreds of millions of dollars in annual aid from the United States and the European Union.

Abiy was awarded the Nobel Peace Prize in 2019, for moves to end long-standing military tensions with neighboring Eritrea. Both Ethiopian and Eritrean officials have denied that Eritrean forces were involved in the Tigray conflict, despite conflicting accounts from witnesses.

In an email on Thursday, Eritrea’s information minister, Yemane Gebremeskel, said he was not aware of the Amnesty report. After the report was published, he disputed its research methods and accused Amnesty of bias against Eritrea.

“Amnesty International have a track record of producing false news on Eritrea,” Yemane said, adding that the new report was “utterly false.”

Amnesty said it shared the report with Ethiopian officials last week but had not received a response. The Ethiopian Embassy in Washington did not respond for a request for comment.

On Thursday, a message on a Twitter account run by the Ethiopian government said the Ethiopian Human Rights Commission was investigating “crimes and incidents” during the conflict.

The message also pointed to allegations of a massacre in the Ethiopian town of Mai-Kadra, which Amnesty also investigated, finding witnesses who said that TPLF-aligned groups were the perpetrators.

“We are deeply troubled by credible reports of human rights violations and abuses in the Tigray region, and we have pressed the Ethiopian government repeatedly to ensure the protection of civilians, including refugees,” the U.S. State Department said in a statement Thursday. “Eritrean forces should be withdrawn from Tigray immediately.”

The statement called for the international community to play a role in preventing rights violations, and for accountability on all sides.

In a phone call with Kenyan President Uhuru Kenyatta on Thursday, President Biden discussed “the deteriorating humanitarian and human rights crises” in Tigray, according to the White House.

Because of ongoing restrictions on access to the Tigray region, Amnesty said its report was based on phone interviews with Axum residents and in-person interviews with refugees in Sudan. Many witnesses provided firsthand accounts of seeing friends and relatives shot by Eritrean soldiers.

“All we could see on the streets were dead bodies and people crying,” one man said of his return to Axum after fleeing. Like all 41 witnesses interviewed for the report, he was not named.

Amnesty confirmed some details in accounts with the use of satellite imagery, including damage consistent with reports of looting by soldiers and the likely location of mass graves.

The conflict in Tigray began in early November after Abiy accused the TPLF, the province’s ruling party, of attacking a military base and attempting to steal military equipment.

The TPLF had been a major political force in Ethiopia for 27 years after overthrowing a communist government in 1991, dominating the ruling political coalition and allowing a small cabal of former fighters from Tigray to control key parts of the military and the economy.

But after Abiy won the 2018 election, TPLF leaders refused to join with the Prosperity Party, a newly formed group that opposed the nation’s previous system of ethnic federalism, and grew increasingly angry at Abiy’s attempts at reform.

The ensuing conflict was bloody, with accusations of war crimes on both sides. The region was already awash with weaponry because of its place on the front lines of Ethiopia’s 1998-2000 war with neighboring Eritrea.

Opposition groups have said the death toll from the Tigray conflict amounts to at least 52,000. More than 61,000 Ethiopians fled into Sudan, according to the U.N. Though the Ethiopian government has said the conflict in Tigray is largely over, accounts of violence continue to emerge.

Before the release of the Amnesty report on Thursday, after a British security research organization released satellite images that it said showed hundreds of buildings deliberately burned around the town of Gijet, the Ethiopian government pledged to investigate.

Before the conflict, Axum was best known for its rich history as the former capital of the ancient Aksumite Empire and as a tourism destination because of its archaeological ruins and Ethiopian Orthodox religious sites.

But as refugees escaped the closed borders of Tigray, reports of a massacre in the city soon spread. In a recent account published by the Associated Press, a deacon at an Orthodox Church said at least 800 were killed on the weekend of Nov. 28-29 and that thousands likely died overall.

Amnesty’s account supports accounts from the Associated Press and others, with reports of house-to-house searches, indiscriminate killing of civilians and mass graves. Witnesses told Amnesty researchers that the massacre started after TPLF-aligned fighters attacked an Eritrean position near the town.

Although Eritrea has not confirmed the presence of its troops, witnesses said that they were easily identifiable by their dialects and the vehicles that said “Eritrea” on them. “Some of them told us they are Eritrean soldiers,” one women told Amnesty.

In a statement earlier this month, the E.U. said the presence of Eritrean troops was “exacerbating ethnic violence” in Tigray. The bloc suspended $109 million of aid to Ethiopia because of the conflict in December.

Eritrean President Isaias Afwerki, who took control of the country in 1992, leads the country as a military dictatorship. Eritrean dissidents say the peace deal has led to few positive developments for the country, which remains repressive.

This report has been updated.