TOKYO (Reuters) - Japan has detected a strain of bird fluin four wild swans after stepping up checks following majoroutbreaks of the disease in neighboring South Korea, local andgovernment officials said on Monday.

The birds, three of which had died, were found on theshores of Lake Towada in Akita prefecture in the north on April21, the prefectural government said in a news release.

Inspectors detected the H5 strain of bird flu in the swans,the prefectural government said, but they were still checkingwhether it was the highly virulent H5N1 strain.

Authorities have patrolled the area but have not found anyincidents of large numbers of deaths or unnatural deaths inwild fowl.

There are no chicken farms within a 10 km radius of thearea where the swans were found, and no unusual incidents werenoted at other farms.

"We've asked to step up surveillance measures at poultryfarms in the prefectures of Aomori, Akita and Iwate," a farmministry official said, referring to the prefectures in thearea.

"Japan has asked poultry farms to strengthen surveillanceafter the case in South Korea in early April," he said.

The official said Japan's last case of bird flu was foundin a wild bird in March 2007 in Kumamoto prefecture on Japan'ssouthern island of Kyushu.

Prior to that, Japan reported cases of bird flu at fourpoultry farms in January 2007.

(Reporting by Chikafumi Hodo and Miho Yoshikawa; Editing byHugh Lawson)