The four casualties were identified as Vu Sa Huynh, 38; Le Hung Vinh, 42; Tran Van Tham, 59 and Duong Van Sau, 38.

Nguyen Trung Kien, 35, survived and was sent to Phu Tho General Hospital until his condition stabilises.

The accident occurred around 5:30pm on Monday.

The factory had outsourced a Hanoi-based environment company to clean up the biogas pit in the plant. During the operation, Tham and Sau, both cleaners, started suffocating and called out for help.

Huynh, Vinh and Kien, working nearby, heard the calls and jumped down the pit to help. But they failed and all five suffocated and passed out.

The factory informed local officials, but when help arrived it was too late, with only Kien surviving.

"An investigation is underway, and we will get to the bottom of this," said Ho Dai Dung, Deputy Chairman of Phu Tho People's Committee.

Miwon Vietnam is a subsidiary of South Korea's Daesang Corporation, specialising in food seasoning products. 

Source: Vietnam News