Taiwanese authorities said Thursday they had slaughtered 9,000 ducks contaminated with the toxic chemical dioxin, but there were fears more had already been sold to consumers.

All ducks at a farm in south Taiwan's Kaohsiung county were killed on Tuesday after they were found to have up to five times the recommended maximum level of dioxin, the Council of Agriculture said.

"The contaminated ducks have been destroyed," Lee Chun-chin, an official at the council, told reporters.

Even short-term exposure to dioxin can affect liver function, according to the World Health Organization.

Official guarantees that no tainted ducks have been sold to consumers failed to ease suspicions among some members of the public.

"It's impossible. I have monitored the duck farm for three years," said Huang Huan-chang, a local community college professor who provided the tip-off that led to the government action, according to a local television report.

Initial investigations of soil around the duck farm indicated it may have been polluted by a nearby illegal furnace slag dump.