Japan on Tuesday confirmed four swans found last week were infected with the H5N1 strain of bird flu.

It was the first case of bird flu in Japan since March 2007 when the highly virulent H5N1 strain was found in a wild bird in Kumamoto prefecture on Japan's southern Kyushu island.

The swans, three of which had died, were found on the shores of Lake Towada in northern Akita prefecture on April 21, the prefectural government said.

Inspectors had initially detected the H5 subtype of bird flu in the dead swans and conducted further tests, the local government said on Monday.

Japan has been stepping up checks of birds after a series of bird flu outbreaks in South Korea over the past month.

There are no chicken farms within a 10 kilometer (6 mile) radius of the area where the swans were found, and no unusual incidents were noted at other farms.

Local authorities plan to conduct on-site inspections on Wednesday and Thursday at 15 farms within a radius of 30 km (19 miles) of the site where the swans were found, the official said.

Earlier on Tuesday, South Korea reported a suspected bird flu outbreak at a chicken farm in Ulsan City which, if confirmed, would be the first in the southeast, as the country grapples with its worst outbreak of avian influenza.

South Korea previously confirmed 20 cases of the H5N1 strain in poultry in less than a month, despite having killed more than 5 million chickens and ducks, as the virus spreads at its fastest rate since the country reported its first case in 2003.

No human deaths from the disease have been reported in South Korea or Japan.