The United Nations has expressed concern about possible war crimes ahead of a threat by the Ethiopian army to start an assault on the northern Tigray region's capital.
Fighting between Ethiopia's central government and forces in Tigray has been going on for almost three weeks.
Hundreds have reportedly been killed and tens of thousands have fled.
Aid groups fear the conflict could trigger a humanitarian crisis and destabilise East Africa.
The UN said it was alarmed by the threat of major hostilities a day before the Ethiopian army said it would advance on Tigray's capital Mekelle, home to about 500,000 people.
On Sunday, Ethiopia's Prime Minister Abiy Ahmed issued a 72-hour ultimatum to Tigray's forces, telling them to surrender as they were "at a point of no return".
But Tigray's forces have vowed to keep fighting, with their leader Debretsion Gebremichael saying they were "ready to die in defence of our right to administer our region".
Meanwhile, Ethiopia's state-appointed Human Rights Commission has accused a youth group from the Tigray region of being behind a massacre earlier this month in which it says more than 600 civilians were killed.
The commission said the group stabbed, bludgeoned and burned to death non-Tigrayan residents of the town of Mai-Kadra with the collusion of local forces.
Human rights group Amnesty International first highlighted reports of a massacre in Mai-Kadra, but was unable to confirm who was behind it, or exactly how many died.
The Tigray People's Liberation Front (TPLF), a political party which controls Tigray, denied involvement, and called for an independent international investigation into the killings.
The conflict started after Ethiopia's central government accused the TPLF of holding an illegal election and attacking a military base to steal weapons.
In response, Abiy - a former Nobel Peace Prize winner - ordered a military offensive against forces in Tigray, accusing them of treason.
The TPLF sees the central government as illegitimate, arguing Mr Abiy does not have a mandate to lead the country after postponing national elections because of coronavirus.
What did the UN say?
UN high commissioner for human rights Michelle Bachelet expressed "alarm at reports of a heavy build-up of tanks and artillery around Mekelle".
She called on all sides to give "clear and unambiguous orders to their forces" to spare civilians.
"The highly aggressive rhetoric on both sides regarding the fight for Mekelle is dangerously provocative and risks placing already vulnerable and frightened civilians in grave danger," Bachelet said.
"I fear such rhetoric will lead to further violations of international humanitarian law."
The rhetoric has been ramped up in recent days. On Sunday, the Ethiopian army said "there will be no mercy" for Mekelle's residents when its soldiers "encircle" the city.
Such talk could constitute a war crime, Bachelet said.
Abiy has repeatedly said the Ethiopian army would protect civilians in its campaign against forces in Tigray.
But Bachelet said a virtual communications blackout in Tigray was making it difficult for the UN to monitor the human rights and humanitarian situation.
"Reports continue to emerge of arbitrary arrests and detentions, killings, as well as discrimination and stigmatisation of ethnic Tigrays," the UN said.
The UN Security Council is due to hold its first meeting on Tuesday to discuss the fighting in Tigray.
At least 40,000 refugees have already crossed into neighbouring Sudan. The UN refugee agency has said it is preparing for up to 200,000 people to arrive over the next six months if the fighting continues.