TOKYO (Reuters) - Japan on Tuesday confirmed four swansfound last week were infected with the H5N1 strain of bird flu.

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

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

Inspectors had initially detected the H5 subtype of birdflu in the dead swans and conducted further tests, the localgovernment said on Monday.

Japan has been stepping up checks of birds after a seriesof 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 unusualincidents were noted at other farms.

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

Earlier on Tuesday, South Korea reported a suspected birdflu outbreak at a chicken farm in Ulsan City which, ifconfirmed, would be the first in the southeast, as the countrygrapples with its worst outbreak of avian influenza.

South Korea previously confirmed 20 cases of the H5N1strain in poultry in less than a month, despite having killedmore than 5 million chickens and ducks, as the virus spreads atits fastest rate since the country reported its first case in2003.

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

(Reporting by Teruaki Ueno; Editing by David Fogarty)