The explosion was reported to authorities at around 11:00 am (0200 GMT) at Hanwha's plant in Daejeon, about 150 kilometres south of Seoul.

A representative of the Daejeon Fire Headquarters told AFP that five people were killed and two others injured, without providing further details.

Hanwha Aerospace is a major South Korean defence manufacturer which produces weapons, artillery systems and aerospace components.

Its Daejeon facility focuses on research and development, including advanced weapons technologies and space-related systems.

The blast may have happened during the cleaning process involved in the production of rocket propellants, Hanwha Aerospace CEO Son Jae-il said at a news briefing.

"During this process, explosive materials could remain on the tools, and there is a procedure to remove and clean those residues. It is presumed that the accident occurred during this stage of the process," he said.

"We will take this accident with the utmost seriousness and fundamentally review and rectify the company's safety systems to ensure that such an incident is never repeated."

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