The 10-day conflict in Tigray region has killed hundreds, sent refugees flooding into Sudan, and raised fears it may suck in Eritrea or force Ethiopia to divert troops from an African force opposing al-Qaeda-linked militants in Somalia.
It may also blemish the reputation of Prime Minister Abiy Ahmed, who won a Nobel Peace Prize for a 2018 peace pact with Eritrea and had won plaudits for opening Ethiopia's economy and easing a repressive political system.
"There is a risk this situation will spiral totally out of control, leading to heavy casualties and destruction, as well as mass displacement within Ethiopia itself and across borders," UN rights chief Michelle Bachelet said via a spokesman.
A massacre of civilians reported by Amnesty International, if confirmed as committed by a party to the conflict, would amount to war crimes, she added.
Prime Minister Abiy Ahmed accuses the Tigray People's Liberation Front (TPLF), which rules the mountainous region of more than five million people, of treason and terrorism.
Federal troops say the TPLF rose against them last week but that they have since survived a siege and recaptured the west of the region. With communications cut and media barred, there has been no independent confirmation of the state of the fighting.
The TPLF says Abiy's government has systematically persecuted Tigrayans since he took office in April 2018 and terms the military operations an "invasion."
Federal troops have been carrying out airstrikes and there has been fighting on the ground since Wednesday of last week. Ethiopia denied a TPLF claim that federal jets had knocked out a power dam.