NAIROBI, Kenya — More than a dozen rights groups have written to the U.N. Human Rights Council urging an immediate halt to “excessive” use of force by Ethiopian security forces.

The letter dated Thursday 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.

“Authorities have also arbitrarily arrested thousands of people throughout Oromia and Amhara during and after protests, including journalists and human rights defenders,” the letter says.

Groups signing the letter include Human Rights Watch, Amnesty International and Reporters Without Borders. The Human Rights Council is expected to convene later this month in Geneva.

This week, the U.S. said it has raised “grave concerns” about the use of force against protesters.

Protests over regional concerns in Ethiopia have turned into broader anti-government protests demanding wider freedoms and the release of detained protesters. Tensions rose over the weekend when more than 20 people were killed during a fire at a prison on the outskirts of the capital, Addis Ababa, which holds many opposition figures and journalists.

The East African country, an ally of the West, is often accused of stifling dissent and cutting off the internet. Thursday’s letter says “there are no effective avenues to pursue accountability for abuses given the lack of independence of the judiciary and legislative constraints.” The ruling coalition party controls all the 547 seats in the federal parliament.

Ethiopian officials have called the recent protests illegal. The country’s prime minister, Haliemariam Desalegn, has ordered security forces to use proportionate measures to control the protests.

As frustration grows, attacks have been reported on some businesses, including foreign ones, with suspected links to the government.