Gunmen wearing military uniforms shot dead a former Burundian security chief and close ally of President Pierre Nkurunziza on Sunday, the presidency said, heightening tensions after a disputed presidential poll.
Nkurunziza called for calm after the attack, which the United States and European Union condemned. But late on Sunday, bursts of sporadic gunfire could be heard in northern Bujumbura.
General Adolphe Nshimirimana, who was in charge of the president's personal security at the time of his death, was killed in a car alongside three of his bodyguards in the Kamenge district of the capital Bujumbura, witnesses said.
They said the four attackers in military fatigues sprayed the car with bullets and drove off shortly after 8 a.m. (0600 GMT). "Two had machine guns and two others rocket launchers. They came in military lorry and returned back in the same car," said a taxi driver named Paul.
Pictures on social media showed a black bullet-riddled SUV with its front tyres flattened and side windows shot out.
Nkurunziza said security forces need to be strengthened to prevent future killings and pleaded with Burundians "not to fall in trap of revenge".
"We ask every Burundian, in hills and the capital, to stay united," Nkurunziza told state radio, vowing the killers would be brought before to justice.
Burundi has been in chaos since late April when Nkurunziza announced he would seek a third term in office, a move that his opponents and Western powers said violated the constitution and a peace deal that ended an ethnically-charged civil war in 2005.
Months of protests and a coup attempt were quelled, but the capital and some areas in the countryside have been rocked by sporadic violence and killings.
Nshimirimana, who was also a former chief of staff in Burundian army, was seen as part of Nkurunziza's inner circle. Opponents say he was one of key men behind the crackdown on protesters and thwarting of the coup.
"I have lost a brother, a companion in the struggle. The sad reality is that General Adolphe Nshimirimana is no longer with this world," Willy Nyamitwe, the presidency's communications chief, said in a message posted on Twitter.
Some of the army generals behind the attempted coup have vowed to lead a rebellion to oust Nkurunziza, who won the July 21 presidential poll after the opposition boycotted the elections.
African leaders fear the violence could split the country down ethnic lines and lead to another civil war, an alarming prospect for a region still scared by the 1994 genocide in neighbouring Rwanda where 800,000 Tutsi and moderate Hutus were slaughtered. Burundi has a similar ethnic make-up.
Western diplomats have also warned a rift in the army could push Burundi back into conflict. The last 12-year civil war pitted the military, which at the time was led by the ethnic Tutsi minority, against rebel factions of the majority Hutus, the biggest of which was led by Nkurunziza.
The European Union urged restraint while U.S. called for calm and "dialogue between all sides".
"No place for violence on path forward," said Thomas Perriello, the U.S. Special Envoy for the Great Lakes Region, on his Twitter account.