CAPE TOWN, South Africa (AP) — An early-morning explosion at an oil refinery in the South African coastal city of Durban caused a large fire and sent plumes of black smoke into the air on Friday.

There were no reports of casualties but South African media, quoting an emergency services official, said six people had been treated for smoke inhalation. The fire spread to a nearby apartment building.

Authorities haven’t said what caused the explosion.

The refinery, which is close to the Durban harbor, is owned by South African oil company Engen. The refinery is listed as the second biggest in the country and has a crude oil refining capacity of 120,000 barrels per day, according to Engen.