A Burundian army officer accused of leading a crackdown on opposition forces was shot dead at army headquarters on Tuesday, military sources told AFP.

Lieutenant Colonel Darius Ikurakure, seen as playing a key role in a wave of disappearances of opposition supporters, was gunned down at lunchtime in the capital Bujumbura by an unknown soldier who escaped.

"It is unfortunately true," said a senior army official, who asked not to be named. There was no immediate official response to the killing.

The army headquarters were practically empty at lunchtime, the sources said.

Hundreds have been killed and almost half a million people have left Burundi since President Pierre Nkurunziza's controversial decision last April to run for a third term.

There have been no claims for Ikurakure's killing so far and it was unclear if the motive was political or personal.

Pacifique Nininahazwe, an activist in exile, said Ikurakure was a "cruel personality at the heart of the crackdown.

"I regret that he has gone without facing justice," he added.

The killing follows similar attacks on senior defence officials.

In September, the chief of staff emerged unscathed from an assassination bid, but in August last year, General Adolphe Nshimirimana -- considered the president's right-hand man -- died in an ambush in the centre of Bujumbura.

Since the July election, violence has become routine but appeared to have abated in March after a rocky February when grenade attacks occurred on a near-daily basis.

But last week, the UN rights chief said reports of torture had increased in Burundi since the beginning of the year and many people there now "live in terror."

UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein said last week that violence in the African country could spiral out of control and take on "massive proportions."

"Continued human rights violations, and impunity for perpetrators, mean that many of Burundi's people live in terror," he said.

"The country remains on the brink of a sudden escalation of violence to even more massive proportions."