Bujumbura (Burundi) (AFP) - Burundi's prosecutor on Sunday said the killers of a powerful general loyal to President Pierre Nkurunziza had been identified, while a rights activist shot in a "reprisal" attack for his murder was allowed to leave the country.

General Adolphe Nshimirimana, widely seen as Burundi's de-facto internal security chief, was killed in a rocket attack on August 2.

He was a close aide to Nkurunziza, whose election for a controversial third term has fuelled violence resulting in about a hundred deaths, a crackdown on protests and an exodus of citizens fleeing the country.

"The identities of the perpetrators are now known," a statement issued by the prosecutor's office said.

"A certain number have been arrested. The rest of them and the masterminds are being sought."

The arrests came as human rights activist Pierre-Claver Mbonimpa was allowed to leave the capital Bujumbura for Belgium on Sunday after he survived an assassination attempt last week, his family said.

Mbonimpa -- who has long criticised the murders of opposition figures -- was shot in the face on August 3 by an unknown gunman on a motorbike.

His daughter Amandine Nasagarare said his family was relieved he was allowed to go. "He will be treated there and they can do all the tests that we can't do here. We are also more reassured about his safety there."

Several diplomats, including the US ambassador and France's charge d'affaires, visited the activist -- who won the Henri Dunant human rights prize in 2011 -- in hospital in Bujumbura.

The attempt on his life was widely seen as a reprisal for the killing of Nshimirimana the day before and has raised fears that the crisis in Burundi could be escalating.

Intense exchanges of gunfire and several detonations were heard over nearly two hours in the capital Bujumbura on Sunday night, an AFP reporter said, but it was not immediately known exactly where the fighting was taking place or who was involved.

Shooting, sometimes accompanied by grenade explosions, has been heard regularly across Bujumbura in recent weeks, but rarely as intensely or for as long as on Sunday.

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Nkurunziza, who has been declared the outright winner of July's controversial election, has given the security forces a week to catch the general's killers.

The prosecutor said a military vehicle had been used by the killers and subsequently burned.

The vehicle had been identified and came from a military camp in the centre of Bujumbura, it added.

According to witnesses quoted in the local media, the attackers were wearing military uniforms.

Nkurunziza's bid to hold onto power was condemned as unconstitutional by the opposition and provoked months of protests in the central African country, where a 13-year civil war only ended in 2006.

In mid-May, rebel generals attempted a coup. They have since launched a rebellion in the north of the country, close to the border with Rwanda.