The United Nations human rights agency said most of those detained in the Ethiopian capital Addis Ababa as well as in Gondar, Bahir Dar and other locations were of Tigrayan origin.

"According to reports, at least 1,000 individuals are believed to have been detained... with some reports putting the figure much higher," spokeswoman Liz Throssell told reporters in Geneva.

The arrests have occurred since Prime Minister Abiy Ahmed's government declared a state of emergency two weeks ago, when Tigray People's Liberation Front (TPLF) fighters threatened to march on the capital.

Lawyers have also said that thousands of Tigrayans have been arbitrarily detained since the announcement of the measures, which allow the authorities to detain without a warrant anyone suspected of supporting "terrorist groups".

Among those arrested since the state of emergency was declared are a number of UN staff.

Throssell said 10 local UN staff as well as 34 drivers subcontracted by the UN were still being held.

"We call for all those still in detention to be immediately released," she said, adding that if that does not happen, "a court or other independent and impartial tribunal should review the reasons for their detention, or they should be formally charged".

She acknowledged that it was "challenging" for the remaining UN rights agency staff to do their work, adding that, "This is why we have reports of at least 1,000 people detained, but we're not in a position to give a more definitive number."

Detention conditions were generally reported to be "poor", she said, with many of those detained held in overcrowded police stations.

Throssell decried that many of those detained had reportedly not even been informed of the reasons for their detention, let alone formally charged.

"We are also concerned at some reports of ill-treatment in detention," she said, adding that while the agency had no specific evidence of torture in detention, this was clearly a concern.

The war between the Ethiopian authorities and the TPLF has over the past year killed thousands and displaced more than two million people, and has left hundreds of thousands in famine-like conditions.

The UN says all sides in the conflict have committed serious human rights violations.