Vietnam is investigating new mass fish deaths along its central coast, an official said Friday, months after a major steel plant was blamed for a toxic leak that wiped out tonnes of marine life in the fishing hub.

Public anger has mounted in Vietnam since dead fish started washing ashore in April after the Taiwanese steel firm Formosa discharged contaminated waste into the ocean, causing the worst ecological disaster in decades.

Formosa was eventually slapped with a $500 million fine following weeks of rare protests in the authoritarian country.

The government is now looking into new reports that boats were seen dumping sludge into waters off of Thanh Hoa province south of Hanoi, where some 50 tonnes of dead fish washed ashore earlier this month.

Officials initially blamed the deaths on blooming red algae.

"An investigation into the cause of the fish deaths is being carried out by experts from the Ministry of Environment and Natural Resources," Cao Thanh Tho, head of aquaculture in the provincial Agriculture and Rural Development department, told AFP.

The Ministry of Natural Resources and Environment said it was analysing waste samples "to determine if there is any link with the fish deaths", with results expected by September 20.

Meanwhile, dead fish, including large carps, also started showing up along the Thu Bon River in central Quang Nam province this week, with state-run Thanh Nien newspaper reporting residents saying the waterway has turned black in recent days.

A Quang Nam official told AFP an investigation was under way.

Earlier this week another Taiwanese firm, Header Plan, was fined more than $9,000 for dumping waste into a canal in Vietnam's southern Dong Nai province, where residents have complained increasing pollution in recent years.