The accident occurred late yesterday in the basement of the Siam Commercial Bank (SCB), one of the country's largest financial institutions.

SCB said contractors had been working to improve the building's chemical fire extinguishers but mistakenly set off the system, releasing chemicals that likely caused oxygen levels to plummet.

Pyrogen manufactures a type of aerosol fire extinguisher that is used in places where putting out a fire with water would damage documents or electrical equipment.

The company says on its website that its aerosol does not deplete oxygen. But it also advises against using it in occupied rooms and says "accidental exposure should be limited to five minutes".

Bangkok's Erawan emergency medical centre said five people were killed at the scene while three died in hospital.

Seven workers were injured and remained in hospital, the centre added.

One picture published by local media showed firefighters wearing masks and handkerchiefs over their faces extracting victims on stretchers from inside a building thick with haze.

Another showed four bodies wrapped in white blankets outside the building.

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