BANGKOK, Thailand - A new outbreak of virulent bird flu was found in ducks in northern Thailand, officials said Monday, the first such case in six months.

The outbreak of the H5N1 virus was confirmed by laboratory tests after the deaths of more than 100 ducks were reported in Phitsanulok province, said Manet Runluang, an official at the Public Health Ministry's Department of Communicable Disease Control.

"We have found the H5N1 virus in the ducks and we have ordered around 2,100 ducks in the area to be killed," said Nirand Uaebumrungsut, a veterinarian with the Agriculture Ministry's Department of Livestock Development.

He added the area has many wild and free-range ducks and the department has been gathering birds from within a three-mile radius of the outbreak to be slaughtered.

Thailand's Public Health Ministry ordered the communicable disease control department to increase measures to curb the outbreak and prevent its potential spread to humans, said Health Minister Mongkol Na Songkhla.

"Although we have not found a case in many months, we have asked governmental health organizations to help monitor cases of flu, coughs and pneumonia, especially among people who have come into contact with birds," Mongkol said.

Health experts advise special caution about flu during winter months, when people are more susceptible to infections.

Since it began ravaging Asia's poultry in late 2003, the H5N1 bird flu virus has spread to the Middle East and Africa and killed at least 159 people around the world, according to the World Health Organization.

There have been 17 human deaths in Thailand.

Most of those killed have been infected by sick birds, but WHO fears the virus could mutate into a form that easily spreads among humans, possibly sparking a pandemic.