BANGKOK : Eight people died and seven were injured when they accidentally triggered a fire extinguisher system inside the vault of a major Thai bank and released a suffocating cloud of chemicals, officials said Monday.

The accident happened late Sunday in a basement security vault at the Bangkok headquarters of the Siam Commercial Bank (SCB), one of the country's largest financial institutions.

SCB blamed the accident on contractors who had been working to upgrade the building's chemical fire extinguisher system but mistakenly set it off, releasing a chemical retardant designed to starve any fire of oxygen.

"The work may have triggered the Pyrogen aerosol which, once it works, will decrease oxygen, that could cause people's injuries and death," the bank said in a statement.

Pyrogen manufactures a type of aerosol fire extinguisher used in places where 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 others died in hospital. Seven workers were injured, four of them critically, the centre added.

Police say seven of those killed were contractors, including one woman. The eighth victim was a bank security guard.

Major-General Porn Sutheerakhun, commander of Thailand's Institute of Forensic Medicine, said initial autopsy results showed the victims suffocated due to lack of oxygen.

"There were no wounds found on their bodies. It was like they were sleeping," he told reporters.

Investigators spent Monday searching for clues to how the fire system was triggered and why victims were seemingly unable to escape the thick cloud of chemicals.

"It's too early to say who is to blame. We need time for experts to investigate the details," acting Bangkok police chief Lieutenant-General Sanit Mahathavorn told reporters.

Another senior officer, Police Major-General Thawatchai Mekprasertkul, said there was no evidence of a fire or explosion that might have triggered the retardant.

Rescue workers had to battle haze to reach victims. One picture published by local media showed firefighters, wearing masks and handkerchiefs over their faces, extracting workers on stretchers from inside a building filled with thick, pale smoke.

Bodies were brought out wrapped in white blankets while paramedics could be seen trying to resuscitate victims on the pavement.

"SCB would like to express its sympathy to those injured and killed," the bank said in its statement, adding it was cooperating with police. The bank said the rest of the building was not affected and was open for business.