Angry Vietnamese protesters rallied in Taiwan Wednesday calling for local conglomerate Formosa to leave their country after an environmental disaster affected hundreds of thousands of people.

The demonstration outside Formosa's headquarters, in Taiwan's capital Taipei, included relatives with families in the area at the centre of the scandal, which saw tonnes of dead fish wash up along Vietnam's central coastline in April.

After weeks of investigation, Vietnamese officials in June laid the blame on Formosa, which is building a multi-billion-dollar steel plant near where the fish died.

The company agreed to pay $500 million in compensation and has apologised for the "environmental incident" which devastated the fishing industry in several central provinces.

Formosa also promised to work with local authorities to help affected people and clean the pollution.

However, Vietnamese media reported recently that local police were now investigating a new incident, after a contractor for Formosa was caught burying hundreds of tonnes of untreated industrial waste.

"We are really angry that the issue of polluting the ocean is not yet resolved and now Formosa are found to have buried toxic waste in many locations," said Nguyen Duc Huy, a Taiwan-based Vietnamese worker whose family live in affected Ha Tinh province.

"Our people are suffering and many lost their jobs because there is no fish in the ocean. The company is really irresponsible. We want it to leave Vietnam and stop polluting our environment," he told AFP.

Around 40 protesters, all Vietnamese currently living and working in Taiwan, held placards reading "Formosa out of Vietnam" and "Damaging the environment is killing people".

Public anger in Vietnam spilled over after the steel plant pollution, with rare protests violently broken up by authorities, who arrested scores of activists.

Formosa has a history of environmental scandals spanning the globe, where the conglomerate has paid millions of dollars in fines over environmental mishaps.

Protesters Wednesday also criticised what they saw as a lack of transparency in the Vietnamese government's handling of the matter, urging Formosa to disclose the investigation report on the pollution and ensure that victims are sufficiently compensated.

A statement read out from campaigning priest Dang Huu Nam, from affected Nghe An Province, who was not at the rally, suggested some in the polluted areas had lost their lives through eating poisoned fish or working in a "toxic environment", without elaborating further.

The only death so far reported by local media as being related to the pollution was of a diver who may have swum in tainted water.