The explosions sowed panic across the city of more than a million people, with a projectile landing close to the small African Great Lakes country's national radio broadcaster, a resident living near the building told AFP, requesting anonymity.

In a video seen by AFP, a tall mushroom cloud of smoke loomed over a Bujumbura neighbourhood at nightfall, which another resident described as "spreading terror" across the city.

Tall flames rising into the sky were also visible in images widely shared across social media, while a Burundian media platform relayed unconfirmed reports of gunfire.

"A serious electrical accident in the ammunition store of the FDNB (Burundi National Defence Force) based in Musaga is the cause of the explosions currently being heard in the economic capital Bujumbura," Burundian army spokesman Gaspard Baratuza said.

Musaga sits in the southern suburbs of Bujumbura, the economic capital of a country ranked by the World Bank as the Earth's poorest by GDP per head in 2023.

"We urge the public to remain calm and avoid the surrounding areas; the relevant services are currently intervening," Baratuza added in a message shared in a WhatsApp group for journalists.

- 'Growing panic' -

A resident of the Gasekebuye neighbourhood, located several kilometres from Musaga, told AFP that "the base camp is sending out munitions".

"It's munitions that are burning. They're sending bombs our way. In my house, some windows have already shattered," the resident told AFP by phone.

The authorities were yet to comment on the incident or give a toll several hours after the incident, about which much remains murky.

SOS Medias Burundi, a platform for independent journalism in the country, warned of "growing panic" in the city from people "fearing a rapid deterioration of the security situation", posting footage of red flames in the distance on X.

"Fear is spreading rapidly among residents, with many continuing to flee their homes," the organisation said, adding it had received reports from residents that "heavy and light gunfire is also ongoing".

It cautioned however that "the situation remains unclear and highly concerning".

Bujumbura sits on the shores of Lake Tanganyika across from the conflict-ridden eastern Democratic of Congo, where Burundi has sent troops to help the Congolese government fight the Rwanda-backed M23 militia.

For years, Burundi has been gripped by a profound economic crisis, notably a three-year-long petrol shortage that has paralysed the country.

Since President Evariste Ndayishimiye took power in June 2020, the former Belgian colony has swung between signs of liberalising a government still in thrall to the country's powerful generals and cracking down on the opposition.

Ndayishimiye had just returned on Tuesday afternoon from a trip to the Central African Republic for his Centrafrican counterpart Faustin-Archange Touadera's third-term inauguration.

Both NGOs and the United Nations have criticised breaches of human rights in the country on his watch.