The victim in South Korea, Pham Van Dao, 53, was a native of the northern city of Hai Phong, Vietnam's Department of Overseas Labor Management told Vietnam Plus on Wednesday.

He was working for the Su Sung Company in Yeongdeok County, 350 kilometers southeast of Seoul.

Dao went to South Korea on an F-1 visa, which allows foreigners to visit and stay with their family. His daughter is married to a South Korean citizen.

The Vietnamese embassy in South Korea has contacted his family to provide assistance.

Korea's Yonhap News Agency reported that Dao and three Thai workers, who were also killed, had entered the three-meter-deep tank of the squid processing factory for cleaning when they inhaled toxic gas and passed out.

They reportedly did not wear safety equipment despite a high risk of toxic gas from decomposed products.

The local police are investigating the cause of the workers' deaths.

The two workers in Japan died also on Tuesday of apparent carbon monoxide poisoning at a demolition site for an apartment building, Japanese media reported.

Another Vietnamese worker has been hospitalized and remains in serious conditions.

Carbon monoxide was detected in a room where the three workers, in their 20s, were found lying on the floor.

The source of the gas has not been determined.