Police are investigating the mystery deaths of five workers at a fermented fish factory in Thailand.

The workers were scrubbing the concrete fermentation tanks when they reportedly suffocated from the noxious fumes at the fish sauce plant in Phetchabun province.

Other employees rushed to pull them out of the pit but also collapsed upon inhaling the toxic vapours in the poorly ventilated facility that was filled with huge jars of rotting fish.

Police Colonel Saranai Kongmuang, commander of the Phetchabun Provincial Police, said officials received a report at 5 pm local time. He arrived with a forensics team to inspect the scene.

The police boss said: 'The pit is a closed system measuring 2 metres in width, 2 metres in length, and 2 metres in depth. Based on initial observations, toxic gas buildup is suspected as the cause of the deaths. However, this is only a preliminary assumption pending further investigation.'

Rescue volunteers installed fans at the location to help disperse the deadly gases before retrieving the bodies.

Footage shows masked officers hoisting the dead workers piled up at the bottom of the fermentation tank.

The deceased were identified by police as Uek Samaklang, 70, Lam Rueangnoi, 76, Chakkrit Senanuch, 50, Suphakrit Chatchiu, 30, and Kunlachat Amnuaymongkhonphon, 41.

Naina Uthityapongsa, 66, a friend of the factory owner, said she called the police after the workers became unreachable by phone in the afternoon.

She said: 'I called the police hotline because my friend said none of the workers were answering their phone. Patrol police went to inspect the factory and found the five men dead inside.'

The bodies were taken to the Lom Sak and Phetchabun Hospitals to undergo post-mortem exams.

Representatives from the Phetchabun Provincial Industry Office inspected the fish sauce factory and said it did not meet industry standards. It was also unregistered in the Ministry of Industry's system, they said.

Fish sauce is a staple condiment widely used in Thai cuisine.

Thailand is a rapidly growing country though it still faces developmental challenges such as outdated infrastructure and poor regulation and law enforcement.