REPUBLIKA.CO.ID, HANOI -- Vietnam on Thursday banned the sale and distribution of non-living aquatic products in its central region, following inconclusive efforts to explain why huge numbers of dead fish washed ashore.

The ban tackles fears of a health threat along 200 km (124 miles) of the coast, after the government said the fish could have been killed by toxic discharge caused by humans or "red tide", when algae blooming at an abnormal rate produce toxins.

On Wednesday, the government said there was no evidence that discharge from a new Taiwanese-built steel plant, a unit of Formosa Plastics, was to blame for the fish deaths.

Vietnam exports $7 billion of seafood a year, mostly from waters off its southern coast, but industry has yet to feel the impact. Fish and shrimp from the affected region are chiefly consumed at home, an industry official told Reuters.

Deputy Prime Minister Trinh Dinh Dung ordered officials in four Vietnamese provinces to seize and destroy dead aquatic products washed ashore or raised along the coastline.

"The use, collection, transport, trading of dead aquatic products as food for people and feed for livestock is strictly prohibited," the order said.

Seafood exports from January to April are estimated to have risen 3.8 percent from a year ago to $1.93 billion, compared to a fall of 15.6 percent in 2015. Vietnam's biggest markets are the United States, Europe and Japan.

On Thursday, Facebook users called for peaceful protests to highlight food security and environmental concerns sparked by the deaths, while a petition on the White House website urging help from the United States drew close to 100,000 signatures.

Signers want President Barack Obama to raise the issue with the government when he visits Vietnam next month, and sought help in doing an independent environmental impact assessment of the steel plant.

Tests of seawater in Thua Thien-Hue, one of the affected provinces, showed higher than normal levels of some pollutants, the province's environment department has said.

Vietnam has a four-day holiday ending Tuesday, and many people had booked tours to beaches in Quang Binh, neighbouring the province of Ha Tinh. The latter, 400 km (250 miles) south of Hanoi, is where dead fish began washing up on April 6.

Seafood markets have been deserted and tourist firms are receiving cancellations, state media reported.

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