CAPE TOWN, South Africa (AP) — Seven people were hospitalized after an early-morning explosion and fire at an oil refinery near the harbor in the South African city of Durban on Friday, emergency services officials said.

The explosion just after 7 a.m. was heard across the city and caused a large fire that sent plumes of black smoke billowing.

Authorities haven’t said what caused the blast.

It's the second explosion at a South African crude oil refinery this year. Two chemical engineers were killed in an explosion at a Cape Town refinery run by a different company in July.

All seven injured people at the Durban refinery were workers at the site, paramedic Garrith Jamieson said. He said six of the workers were being treated for smoke inhalation and the other injured their hip in a fall while trying to escape the fire.

Jamieson said that fire engines, bomb squad units, metro police and search and rescue personnel were on the scene.

Emergency services spokesman Robert McKenzie said the fire was now out. He said there had also been a fire at a nearby apartment complex which fire services also put out. Mckenzie said it was unclear if the two fires were connected. No one was hurt at the apartment complex, he said.

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

Engen said a fire burned at the refinery for about an hour and 35 minutes but didn't mention an explosion. The cause of the fire was under investigation, the company said.

Engen is majority owned by Malaysian oil and gas company Petronas.

The Cape Town refinery hit by an explosion and fire in July is the third largest in South Africa and is run by Astron Energy, which is majority owned by Anglo-Swiss company Glencore plc.