Eight people have died at the headquarters of one of Thailand's largest banks after workers accidentally released fire-extinguishing chemicals during a safety system upgrade.

The accident happened in the basement of the Siam Commercial Bank's headquarters in Bangkok. Seven other people were injured.

Bangkok's Erawan medical emergency centre said the apparent cause of death was suffocation from insufficient oxygen.

Workers activated a system that mistakenly released Pyrogen aerosol, a chemical fire retardant which is normally intended to deprive fire of oxygen, the bank said. A security guard and seven contractors died.

The bank said an initial investigation found the accident was caused by "negligence on the part of our contractors hired to improve fire protection in the building".