A huge explosion at South Korea's third-largest oil refinery has left at least one person dead and nine more injured on Thursday evening.

Footage taken on May 19 shows a huge fireball erupting from the S-Oil facility in Ulsan City.

Authorities deployed 39 fire trucks and 120 firefighters to extinguish the inferno, which reportedly erupted while workers were testing the refinery's alkylation processing unit after maintenance.

The injured have been taken to the hospital for second-degree burns, they added. One person died in the blast.

S-Oil CEO Hussain A. Al-Qahtani said in a statement: 'I would like to express my deepest condolences and apologies to the deceased who lost his life from an incident in S-OIL Onsan Refinery and to his family.

'My apologies also go out to the injured workers and local residents close to the refinery to whom we've caused discomfort.'

The plant has been shut down while a probe is underway to identify the exact cause of the accident.