Burundi, a tiny nation still reeling from a civil war that killed more than 250,000 people, is awash in weapons but attacks like the one Sunday night are rare. Still, the region borders eastern Congo, which is wracked by violence from a myriad of rebel groups.
Bujumbura province governor Jacques Minani said the attackers targeted the pub in Gatumba, west of Burundi's capital, after crossing the river from Congo.
Survivor Jackson Kabura, who was shot in the stomach, said the men entered wearing fatigues.
"I saw a man in military fatigues," he said. "One of them said, 'kill them all, kill them all. Make sure there's no survivors."'
President Pierre Nkurunziza has declared three days of mourning and gave the government one month to find the culprits.
Burundi's war started in 1993, when Tutsi paratroopers assassinated the country's first democratically elected president, a Hutu. A cease-fire was declared in 2006 but it took several more years to finally see an official end to the fighting.
Gatumba was the site of a 2004 massacre of Congolese refugees. Human Rights Watch said at least 150 people were killed in the attack claimed by the country's last rebel army, the Forces for National Liberation.