A Burundian army general who was a close ally of President Pierre Nkurunziza has been killed in the country's capital, Bujumbura. The slaying comes after Nkurunziza won a third term in controversial elections.

General Adolphe Nshimirimana was killed in a car alongside his driver in the capital's Kamenge district, the president's office said Sunday. The general formerly served as chief of intelligence, and was widely seen as a key member of the president's security team.

"I have lost a brother, a companion in the struggle. The sad reality is that General Adolphe Nshimirimana is no longer with this world," the presidency's communications chief, Willy Nyamitwe, said on Twitter.

Witnesses said men in military uniform had ambushed the general's car early in the morning and sprayed it with bullets.

"Two had machine guns and two others rocket launchers. They came in military lorry [sic] and returned back in the same car," a taxi driver told Reuters.

Ongoing violence

Burundi has been wracked by instability since late April when Nkurunziza announced his intention to seek a third term in office, breaking the two-term limit agreed under a peace deal that ended the civil war in 2005. In the past few months there have been deadly protests, a coup attempt and sporadic killings across the country.

Nkurunziza was declared the winner in the presidential elections a little over a week ago. Some of the army generals behind the attempted coup have vowed to lead another rebellion to oust him. A source in the presidency told news agency AFP the situation in Burundi was "serious" and warned of possible revenge attacks.

"The general was somebody who was essential in the system," the source said. "We are trying to manage the situation but it is not easy. Our boys want to take revenge."

There are fears the tensions - as well as possible divisions within the army - could split Burundi down ethnic lines, pushing the central African nation back into conflict.

The last civil war there pitted the military, at the time led by ethnic Tutsi minority, against factions of the majority Hutus - the biggest of which was led by Nkurunziza. At least 300,000 people died in the 12-year war.