TOKYO (AFP) - A dead swan tested positive for bird flu in northern Japan on Monday, the environment ministry said, less than a week after officials announced the first detection of the deadly virus in 13 months.

"The virulent H5N1 type of avian influenza was detected" in the swan found dead on April 24 in the northernmost prefecture of Hokkaido, an official said.

Ministry officials and the Hokkaido government were inspecting chicken farms within a 30-kilometre (19-mile) radius of where the bird was found.

The virus was also found in three swans in northern Akita prefecture in late April, the first detection of H5N1 in Japan in 13 months.

The H5N1 strain has killed more than 230 people worldwide since late 2003, with about half of the cases occurring in Indonesia. There have been no human cases in Japan.

Health experts fear the strain could mutate into a form easily transmitted from person to person, leading to a pandemic.