NAIROBI (Reuters) - 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 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 neighbouring Rwanda.

Till 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.

“Alarmed by violence in Bujumbura today. Need for immediate cease-fire and calm,” Thomas Perriello, the U.S. Special Envoy for Africa’s Great Lakes region, said on his Twitter feed.

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.

Night-time gunfire and sporadic blasts have become common in Bujumbura during a crisis set off in April by Nkurunziza’s bid for a third term, which opponents - often known as ‘Sindumuja’ - said violated a deal that ended the civil war. Nkurunziza won a disputed election in July.

But residents said Friday’s flare-up was much more intense.

“Sindumuja tried to attack military camps but they failed,” presidential media adviser Willy Nyamitwe wrote on Twitter, describing the raids as “a diversion” to try to free prisoners.

“Situation is returning to normal as firearms are seized, many Sindumuja assailants killed or arrested,” he wrote.

FEARS ARMY COULD SPLINTER

Army spokesman Gaspard Baratuza told state radio 12 of the insurgents were killed and 20 were detained after trying to storm military bases to seize weapons. He said five soldiers were wounded. “The situation has now normalised,” he added.

Earlier, a soldier who had spoken to colleagues inside a base in Bujumbura’s northern Ngagara district told Reuters at least two soldiers were killed. An officer at sites in the south of the city said three soldiers were killed in those locations.

The soldiers asked not to be identified.

Kenya Airways, RwandAir and Ethiopian Airlines all cancelled their flights to Burundi. The Kenyan carrier said there were no personnel to allow them to fly. But Nyamitwe said the airport was still open despite the cancellations.

Burundi’s political crisis led to the failed coup in May. One of the generals behind that attempt said in July the group still aimed to topple the president. Other plotters were caught and are due to face trial in the town of Gitega from Dec. 14.

Experts voiced fears that the army, which was restructured after the civil war to include both professional troops and rebel fighters, might fracture, igniting a broader conflict.

A deputy presidential spokesman wrote on Twitter that the cabinet was meeting as scheduled on Monday and would discuss the 2016 budget, but not the idea of declaring a state of emergency. He said the aim of the “armed gang” was to free prisoners.

Alongside the raid on Ngagara camp, two southern sites were attacked - ISCAM, which is a higher institute for officers, and a camp next to it, known as BASE, soldiers said. Muha camp nearby was not a target despite earlier reports, they said.

In the attacks on southern sites, a soldier said the assailants, some of them wearing military uniforms, first targeted a police station near ISCAM. At the same time, the attackers launched an assault on BASE, said the soldier.