Heavily-armed gunmen launched coordinated assaults on three army bases in the Burundi capital on Friday leaving at least a dozen attackers dead, the military said, in the worst unrest since a failed May coup.

Army spokesman Colonel Gaspard Baratuza claimed that 12 attackers were killed and 21 captured while five soldiers were wounded, following the early morning assaults on a base at Ngagara and a military training college in the capital, as well as another base at Mujejuru, 40 kilometres (25 miles) outside the city.

Baratuza said the attackers aimed "to stock up on weapons and ammunition".

"The army has defeated them seriously," the spokesman said, while Bujumbura residents reported continuing gunfire into the afternoon.

Clashes continued throughout the day in different parts of the capital with witnesses describing heavy firing, including artillery, lasting several hours at the military locations.

Streets were deserted as city residents stayed home and the army and security forces attempted to lockdown the city. Witnesses reported many arrests by security forces of mainly young residents of different city neighbourhoods.

US Special Envoy for the Great Lakes Region, Thomas Perriello, said he was "alarmed" by the violence and called for an "immediate ceasefire and calm".

Friday's firefights are the heaviest since a failed coup in May, sparked by President Pierre Nkurunziza's bid for a third term, which he later won in disputed elections in July.

Months of street protests have devolved into regular armed attacks with gunfire disrupting the nights and dead bodies appearing on city streets almost every day.

Attacks on security forces have escalated, with frequent ambushes of police convoys by rebels armed with assault rifles, rocket-propelled grenades and mortars aimed at government installations.

Eight months of unrest have claimed the lives of at least 240 people so far while more than 200,000 have left for neighbouring countries, according to the UN.

Frightened residents of Bujumbura said Friday's fighting was the worst in months.

"I am holed up in the corridor of my house with my wife and children," said Eric, a resident of Musaga. "Pray for us because we will die!"

As sporadic gunfire continued around Bujumbura, a government spokesman claimed on Twitter that the attacks had been foiled.

Referring to the gunmen as "Sindumuja" -- meaning "I am not a slave", a name sometimes used for the insurgents -- presidential media advisor Willy Nyamitwe wrote, "Tonight the #Sindumuja tried to attack military camps but they failed."

"A failed coup in May 13, many defeated attacks including this one... all their plans against #Burundi fail," he added later.

Regional carriers Ethiopian Airlines, Kenya Airways and RwandAir all cancelled scheduled flights to Bujumbura on Friday.

The UN, aid agencies and several foreign embassies urged their employees to stay at home and lock their doors.

"This is a very worrying escalation because it is genuine military, or paramilitary, operations taking place in the capital," said one European diplomat.

Burundi is still scarred by memories of its 1993-2006 civil war pitting rebels from the Hutu majority against an army dominated by minority Tutsis.

Some 300,000 people were killed in the conflict, which began a year before a genocide of mainly Tutsi people in neighbouring Rwanda.

Although the unrest and armed battles are political, the UN and others observers have raised fears that the country might yet split along ethnic lines as in the past.