A major outbreak of avian flu in Taiwan has spread to 19 more farms with a total of 160,000 birds slaughtered in the island's worst bout of the disease in a decade, authorities said Wednesday.

The number of poultry farms infected with the virus has almost doubled since Tuesday, jumping from 21 to 40. More than 10,000 geese have been killed since Tuesday afternoon.

"We have been adopting stringent measures so that we can limit the further spread of the outbreak as soon as possible," Chang Su-san, head of the Bureau of Animal and Plant Health Inspection and Quarantine, told reporters Wednesday.

She would not specify how many more birds would be slaughtered but said that samples had been taken from another 47 suspect farms where around 340,000 birds are kept.

The outbreak is a new variant of the H5N2 and H5N8 strains of the disease which are not deadly to humans.

Agriculture minister Chen Bao-ji has said he expected the number of infected farms to keep growing and more birds to be culled.

The first outbreak was confirmed at a chicken farm in southern Pingtung county last week then at a goose farm in the northern city of Taoyuan on Tuesday, leading to the slaughter of 1,900 birds.

Most infected farms are in the south, which is home to the majority of the island's goose and duck farms.

Experts say the outbreak is likely to have been caused by migratory birds bringing the virus onto farms, which have been told to strengthen their nets to prevent contact between poultry and wild birds.

The authorities have adopted stringent measures to stop the outbreak including a ban on transportation of the birds at the suspect farms and a Tw$1 million ($31,250) fine for farmers if caught dumping infected poultry.

Angry farmers have accused the authorities of attempting to cover up the outbreak and being too slow to respond, an allegation they have denied.

Taiwan has reported several outbreaks of H5N2 but has no recorded cases of the potentially deadly H5N1 strain, although authorities said pet birds smuggled from China tested positive for the strain in 2005 and 2012.

Taiwan authorities in 2004 slaughtered 467,000 birds after H5N2, a less virulent form than the H5N1 strain deadly to humans, was discovered in farm chickens.