The letter also calls for an independent investigation into the reported killings of hundreds of people in Ethiopia’s Amhara and Oromia states since November 2015 amid protests.
The Human Rights Council is expected to convene later this month.
Groups signing the letter Thursday include Human Rights Watch, Amnesty International and Reporters Without Borders.
Protests on regional concerns have turned into broader anti-government protests demanding wider freedoms. Ethiopia, an ally of the West, is often accused of stifling dissent and cutting off the internet.
Ethiopian officials have called the protests illegal.
The U.S. has raised “grave concerns” about the use of force against protesters.