"We... are profoundly concerned by recent reports of the Ethiopian government's detention of large numbers of Ethiopian citizens on the basis of their ethnicity and without charge," the joint statement issued by the US State Department said.
Australia, Canada, Denmark, the Netherlands, and the United Kingdom joined the United States in urging the Ethiopian government to "cease immediately" the arrests, saying "many of these acts likely constitute violations of international law."
"Individuals are being arrested and detained without charges or a court hearing and are reportedly being held in inhumane conditions."
The statement cited reports by the Ethiopian Human Rights Commission (EHRC) and Amnesty International, which "describe widespread arrests of ethnic Tigrayans," including the elderly and young children.
The war in northern Ethiopia broke out in November 2020 when Prime Minister Abiy Ahmed sent troops to topple the Tigray People's Liberation Front (TPLF) -- a move he said came in response to TPLF attacks on federal army camps.
Though Abiy, the 2019 Nobel Peace Prize winner, promised a quick victory, by late June the TPLF had retaken most of Tigray, and it soon launched offensives into neighboring Afar and Amhara regions.
As fighting has dragged on, the humanitarian toll has spiked, with reports of massacres and mass rapes.
The UN undersecretary for humanitarian affairs Martin Griffiths last week said the conflict in Ethiopia has sparked perhaps the world's most worrying humanitarian crisis and expressed deep concern for the stability of a nation of 115 million people composed of more than 80 ethnic groups.
Monday's joint statement reiterated the countries' "grave concern at the human rights abuses and violations, such as those involving conflict related sexual violence" reported by the UN high commissioner for human rights and the EHRC, and at "ongoing reports of atrocities" committed by all sides.
The nations called again for a ceasefire, international monitors to be allowed access to the country, and for neighboring Eritrea to withdraw forces sent to back the government -- a move that prompted Washington to slap new sanctions on the country in November.