The United Nations human rights chief, Michelle Bachelet, on Friday expressed "increasing alarm" at the situation in Tigray state, Ethiopia, saying it risked spilling completely out of control.
Mass killings reported by Amnesty International, if confirmed as having been committed by a party to the conflict, would amount to war crimes, she added.
"... there must be an independent investigation and full accountability for what has happened," she said via a spokesman.
Ethiopian police visited an office of the United Nations' World Food Program (WFP) in Amhara region to request a list of ethnic Tigrayan staff, according to an internal UN security report seen by Reuters on Friday.
The local police chief informed them of "the order of identifying ethnic Tigrayans from all government agencies and NGOs", the report said, underlining fears over the ethnic undertones to federal troops' push against the leaders of Tigray province in north Ethiopia which borders Amhara.
Ethiopia sought to tighten its grip on the rebellious Tigray region on Friday by appointing a new local leader during a military offensive that has killed hundreds and shaken the wider Horn of Africa region.
Federal troops' 10-day push against the northern state has sent refugees flooding into Sudan.
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.
The conflict began last week when Abiy says the TPLF attacked a military base, since when his army has been carrying out airstrikes and ground combat which the government says has "liberated" west Tigray.
Abiy, who comes from Ethiopia's largest ethnic group the Oromo, said parliament had named former Addis Ababa university professor and deputy minister for science and higher education Mulu Nega, 52, as chief executive of Tigray.
"The Chief Executive will recruit and appoint heads to lead executive organs of the regional state from political parties legally operating in the region," he tweeted.
