Ethiopia's state-appointed rights group accused a Tigrayan youth group on Tuesday of massacring hundreds of civilians as federal and local forces both claimed advances in a three-week war in the country's mountainous north.
Prime Minister Abiy Ahmed's government said enemy soldiers were surrendering as it advanced towards the regional capital, but the Tigrayans reported they were resisting and had destroyed a prestigious army division.
The Ethiopian Human Rights Commission published findings into a November 9 attack in Mai Kadra in southwest Tigray state - first reported by Amnesty International - where it said a youth group called Samri killed at least 600 people of the minority Amhara and Wolkait ethnic groups in the town.
Non-Tigrayans were beaten to death, stabbed, set on fire and strangled with ropes, the report said, though some residents protected neighbors by hiding them in homes. The commission accused local forces of colluding in the massacre.
The Tigray People's Liberation Front (TPLF) was not immediately available but has previously denied involvement.
Reuters has been unable to verify statements made by either side since phone and internet connections to Tigray are down and access to the area is strictly controlled.
Since fighting began on November 4, hundreds have died, more than 41,000 refugees have fled to Sudan, and there has been widespread destruction and uprooting of people from homes.
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The war has spread to Eritrea, where the Tigrayans have fired rockets, and also affected Somalia where Ethiopia has disarmed several hundred Tigrayans in a peacekeeping force fighting Al- Qaida-linked militants.
Abiy's government said many Tigrayan combatants had responded to an ultimatum to lay down arms before a threatened offensive against Mekelle city, with half a million inhabitants.
The deadline expires on Wednesday.
"Using the government's 72-hour period, a large number of Tigray militia and special forces are surrendering," a government taskforce said.
The battle-hardened TPLF, which had ruled the region of more than 5 million people, gave a different version, saying their troops were keeping federal forces at bay and scoring victories.
Their spokesperson Getachew Reda said an important army unit - which he named as the 21st mechanized division - was destroyed in an assault at Raya-Wajirat led by a former commander of that unit now fighting for the TPLF.
The prime minister's spokesperson Billene Seyoum denied that.
TPLF leader Debretsion Gebremichael has disputed the government version that Mekelle is encircled at a roughly 50km (30 mile) distance, telling Reuters the ultimatum was a cover for government forces to regroup after defeats.
The United States - which regards Ethiopia as a powerful ally in a turbulent region - France and Britain were the latest foreign powers to call for peace.
Washington backed African Union (AU) mediation efforts "to end this tragic conflict now", while Paris and London warned against ethnic discrimination.
The UN Security Council was to hold informal talks later on Tuesday over Tigray, according to a UN source and an email.
Abiy, who won the Nobel Peace Prize last year for ending a standoff with Eritrea, has said he will not negotiate with the TPLF though he does plan to receive AU envoys.
His predecessor, Prime Minister Hailemariam Desalegn, criticized international mediation efforts by "well-intentioned outsiders" that he said obscured the crimes of the TPLF and overestimated their importance in Ethiopian society.
"The key problem in the international community's approach to Ethiopia is the assumption of moral equivalence, which leads foreign governments to adopt an attitude of false balance and bothsidesism" between the federal and Tigrayan sides, he wrote in Foreign Policy magazine.
Abiy, whose parents are from the larger Oromo and Amhara groups, denies any ethnic overtones to his offensive, saying he is pursuing criminals who ambushed federal forces.
The TPLF says he wants to subdue Tigray to amass power.
Since taking office in 2018, the prime minister has removed many Tigrayans from government and security posts and arrested some on rights abuse and corruption charges, even though he was their former military comrade and coalition partner.
The conflict threatens to destabilize the vast nation of 115 million people from myriad ethnic groups whose struggles for greater resources and power intensified when Abiy took office.
In Geneva, the UN human rights chief voiced alarm over reports of tank and artillery build-ups outside Mekelle.
"We have seen an Ethiopian colonel come out and say there will be no mercy. On the other side you have had the TPLF leadership say they are ready to die," said Michelle Bachelet.
"This is the kind of rhetoric that is extremely worrying and that may provoke or may lead to serious violations of international humanitarian law."