TAIPEI (REUTERS) - Activists in Taiwan have called on the island's largest industrial group, Formosa Plastics, to investigate recent mass fish deaths in Vietnam, near where the company has a steel plant.

The deaths in April of fish in Vietnam's Ha Tinh province, where the Taiwan group's US$10.6 billion (S$14.3 billion) steel plant is located, and three other provinces along a 200km stretch of coast, sparked rare protests in Communist-ruled Vietnam.

A preliminary Vietnamese investigation found no link between the plant and the mysterious fish deaths.

Vietnam has invited experts from Germany, Japan, the United States and Israel to inspect the Ha Tinh site in an attempt to find the cause. They have yet to announce any findings.

Echo Lin, an activist shareholder in one of the group's companies, said the company should investigate the disaster.

"It is their responsibility to prove that they are innocent," Lin told reporters outside the annual meeting of Formosa Plastics Corp, the conglomerate's flagship firm. "That's why we ask them to conduct an investigation and clarify if it is not related to the company."

A small group of protesters, including Vietnamese workers in Taiwan, rallied outside the hotel where the company held it meeting. Some held signs reading "I love the beach; ruining the environment is a sin".

During the meeting, Lin, who is secretary-general of the Environmental Jurists Association in Taiwan, asked the company's senior management to investigate independently and disclose its findings.

Formosa Plastics Corp Chairman Jason Lin told shareholders the group had asked to take part in the investigation being led by Vietnam and it was awaiting the international inspectors'findings. "Formosa Plastics Corp is only investing. We do not participate in its management," Lin said of the steel plant, adding that he could not speak on its behalf.

Formosa's steel plant project is one of the largest foreign direct investments in Vietnam. The plant is 70 per cent owned by companies in the Formosa Plastics group, while Taiwan's China Steel Corp controls 25 per cent and Japan's JFE Steel Corp holds 5 per cent.

Taiwanese lawmakers have begun to take notice of the case on concern it may become a foreign relations problem as the new Taiwan government steps up efforts to deepen trade and economic ties with South-east Asia.