The agricultural ministry said a suspected outbreak was discovered on a chicken farm in western Okayama prefecture, and tests were now underway to confirm if it is the third case in Japan this month.

The new case came hours after the ministry said test results confirmed an outbreak in Hyuga town in southern Miyazakito prefecture was caused by the deadly H5N1 virus.

At least 1,300 chickens were reported dead on the farm, and the government has ordered the culling of the remaining 52,500 chickens.

A 10-kilometer quarantine zone would also be thrown up.

Another outbreak has reported in Miyazaki on January 13 and also later confirmed as the H5N1 strain. Some 3,900 chickens were found dead, prompting authorities to slaughter the remaining 8,100 birds to contain the virus.

The agricultural ministry and local authorities are investigating if the two Miyazaki cases are linked.

In the Okayama case, 17 birds were reported dead on Friday and Saturday on the farm that keeps 12,000 birds, the ministry said.

"We will conduct detailed testing to see what caused the deaths," a ministry official told AFP.

The virulent H5N1 strain has killed about 160 people worldwide since late 2003 and can be transmitted through contact with infected birds' waste.

Health officials have warned that if the disease mutated into a form easily transmissible by humans, it could cause a pandemic with the potential to kill millions of people.

Japan confirmed an outbreak of the H5N1 strain of bird flu in January 2004. Since then, the nation saw several cases of outbreaks with the H5N1 strain as well as outbreaks with the less serious H5N2 virus.