Ethiopia on Tuesday accused Eritrea of committing "mass killings" during the Tigray war of 2020-2022 at a time when the two countries were allies.
The Tigray civil war killed at least 600,000 people and a peace deal never fully resolved the tensions, with fighting breaking out in two areas last week as flights were suspended for five days.
Rights groups have documented widespread war crimes and crimes against humanity committed during the war, including the systematic use of rape.
Relations between Ethiopia and Eritrea have deteriorated since they fought together against the Tigrayan rebels during the conflict.
Ethiopian Prime Minister Abiy Ahmed told parliamentarians that Eritrea "carried out mass killings" in the Tigrayan town of Axum and elsewhere.
"When we entered Adwa, they looted factories and dismantled their parts, and the conflict escalated. In Adigrat... they destroyed pharmaceuticals and looted," he said, referring to other towns in the region.
It is believed to be the first time Abiy has made such accusations against Eritrea.
Eritrean Information Minister Yemane Gebremeskel described his allegations as "cheap and despicable lies".
Abiy "and his top military brass were profusely showering praises and state medals to the Eritrean army and its senior officers until yesterday -- both during the war and long after its end," he told AFP.
Eritrean President Isaias Afwerki, who has been in power since 1993, has repeatedly rejected accusations of human rights violations in Tigray, denouncing them as "disinformation" in a February 2023 speech.
The peace deal that ended the Tigray civil war, known as the Pretoria Agreement, has never been fully implemented.
Fighting broke out again last week in a disputed area of western Tigray called Tselemt and the Afar region to the east of Tigray.
Flights to the region were suspended from Thursday to Tuesday due to the conflict, although the government has yet to comment on the clashes.
The international community fears the fighting could turn into an international conflict between Ethiopia and Eritrea.
Eritrea, one of the world's most closed countries, gained independence in 1993 after decades of armed struggle against Ethiopia.
They later fought a 1998-2000 border war, killing tens of thousands.
Abiy initially sought rapprochement with Eritrea when he came to power, earning him the Nobel Peace Prize in 2019.
But they disagreed over the Tigray peace deal, and Eritrea accuses Ethiopia of seeking to seize its port at Assab as part of the landlocked country's efforts to gain sea access.
Last month, Afwerki said Ethiopia's ruling party had "declared war".
For its part, the Ethiopian government accuses Eritrea of forging closer ties with the Tigrayan authorities that they once fought.
