Vietnamese police have launched an investigation into illegal dumping of waste material from a Taiwanese steel company already under fire for massive fish deaths in what officials say was the country's worst environmental disaster.

Bui Dinh Quang, deputy police chief in Ha Tinh province where a unit of the Formosa Plastics Group is located, said a local company was the target of the investigation after police last month caught the company burying the industrial waste at a private farm.

He says tests confirmed the waste contained harmful levels of cyanide.

Quang says the local company was not authorized to handle harmful industrial waste.

Formosa acknowledged in June that it was responsible for the fish-death pollution and pledged to pay $500 million to clean it up and compensate affected people.