Five people in Japan may have been infected with the bird flu virus after an outbreak among chickens in western Japan in February, Kyodo news agency said, adding that none was in serious condition.

If confirmed the cases would mark the first human infections of bird flu in Japan, which reported several cases of bird flu earlier this year.

All of the cases reported in Japan have been identified as the H5N1 virus, the strain that has hit other countries in Asia and has been blamed for human deaths in Vietnam and Thailand.

"An antibody to the bird flu virus has been detected but the virus itself was not found in blood serum samples taken from the people," Kyodo said in the report, quoting sources familiar with the case.

"They all had fever but none of them was in serious condition, according to the sources," Kyodo said.

Four of those possibly infected were workers at a farm in the town of Tamba in Kyoto prefecture, western Japan, Kyodo said.

The other person had helped disinfect the farm after a bird flu outbreak among chickens in February, it added.

Other Japanese media including major dailies carried similar reports. Health Ministry officials were unavailable for comment.

After the February outbreak some 240,000 chickens and 20 million eggs were disposed of at the Kyoto farm and another one nearby to prevent the infection from spreading, the agency said.