Gunmen stormed into a bar in Burundi's capital, Bujumbura, late Saturday, killing at least nine people.
Witnesses said the attackers ordered people sitting outside to come inside the building; then opened fire, killing seven instantly. Two died later from their wounds.
Witnesses told the French news agency AFP the gunmen wore police uniforms.
International concern is growing about the increase in violence in the central African country, following President Pierre Nkurunziza's election to a third term in office.
State Department spokesperson John Kirby said Saturday the United States is "particularly concerned... inflammatory rhetoric" by some government officials and a planned security crackdown this weekend are increasing the risk of mass violence in Burundi.
The government has ordered people to hand in their weapons or face consequences.
U.S. President Barack Obama says he plans to remove Burundi from a U.S. trade preference program because of the worsening crackdown on the political opposition there, after Nkurunziza's return to office.
The decision by Nkurunziza to run for a third term sparked anger from many Burundians who said he was violating the constitution and the Arusha Accord that ended the country's 13-year civil war.
Obama said the violence in Burundi "worsened significantly" during Nkurunziza's election campaign.
Clashes between police and protesters, and fear of unrest has prompted some 200,000 Burundians to flee the country, with most going to Tanzania or Rwanda.
Human Rights Watch says "Burundi seems to be descending into uncontrolled violence. A frightening lawlessness is taking hold, which some authorities appear to be taking advantage of to justify brutal repression. Politically motivated killings are more frequent by the day, with hardly any of the killers arrested of prosecuted."
Some material for this report came from AP and AFP.