More than 100 tonnes of clams have perished in central Vietnam, state media reported Thursday, as public outrage mounts over a possible toxic leak into the sea near an industrial zone.

Piles of dead molluscs have been found in the same coastal region where dead fish began washing up on beaches earlier this month, sparking alarm and hammering the local fishing economy.

Clam farmers in Ha Tinh province wept over their staggering losses that occurred just ahead of harvest time, state-run Tuoi Tre News reported.

"We had held a lot of hope about the (clam) harvest," local farmer Tran Thi Lua reportedly said. "Many traders had even paid a deposit to buy my clams, but now all hopes have been shattered."

Vietnamese authorities have banned the trade and consumption of the clams while their investigation is ongoing, as concerns grow that toxic chemicals have leaked into the water from nearby industrial plants.

"To ensure environmental hygiene, food safety and to protect people's health, the Prime Minister asked ministries and localities to... collect and immediately deal with the dead aquaproducts," the government said in a statement late Wednesday.

The area in Ha Tinh province is home to a large steel mill run by a Taiwanese conglomerate, Formosa.

The company has a long history of environmental scandals spanning across the globe.

A government investigation has not yet linked Formosa or other factories in the region to the incident, according to deputy environment minister Vo Tuan Nhan.

But a gaffe by a Formosa public relations official after the fish deaths has heaped pressure on the firm after he said locals must decide whether they valued marine life or foreign investment more.

"You cannot have both," Chou Chun Fan, the company's external relations manager in Vietnam, told state media.

The comments sparked a tide of ire in Vietnamese media and on Facebook -- one of the few avenues for expression in the authoritarian country.

Formosa later apologised for the comments.

In a separate statement over the fish deaths the company said: "So far there is no evidence showing that the fish deaths are linked to our side."

Last year, the country earned $6.6 billion from seafood exports.