Gunmen attacked military sites in Burundi’s capital on Friday and 12 of the assailants were killed while 20 were arrested after heavy fighting, the Army said.

Soldiers told Reuters at least five of their number were also killed, but an official Army spokesman said they were only wounded in the latest flare-up in a nation that Western powers fear may be sliding back into ethnic conflict.

The sound of firing echoed across the capital Bujumbura throughout Friday after heavy gunfire and blasts erupted in the early hours. Residents said streets were empty and police were out in force at a time when people normally head to work.

The outbreak of violence, the worst since a failed coup in May, is unnerving for a region that remains volatile two decades after the genocide in neighboring Rwanda.

The U.N. Security Council was briefed on the developments in Burundi late on Friday. U.S. Ambassador to the United Nations Samantha Power, who is president of the council for December, said the 15-member body was ready to consider “further steps.”

“The members of the Security Council demand that all armed groups put aside their arms and cease all forms of destabilizing activities in order to end the cycle of violence and retaliation,” Power told reporters.

Until now, battle lines in Burundi’s crisis have followed the political divide. But Western powers and regional nations fear old ethnic rifts could reopen.

The United States said it was concerned by the fighting and the African Union called for dialogue.

The United States called for the U.N. Human Rights Council in Geneva to hold a special session on Burundi next Thursday. The call got the required backing of one-third of the body’s 47 members so the session will go ahead, the United Nations said.

Burundi’s 12-year civil war, which ended in 2005, had pitted rebel groups of the Hutu majority, including one led by current President Pierre Nkurunziza, against what was then an Army led by the Tutsi minority. Rwanda has the same ethnic mix.