Japan's health ministry said Tuesday it plans to inoculate thousands of health officials with a government stockpile of vaccines to prepare for a possible bird flu pandemic in humans.

Under the plan, pending approval Wednesday, about 6,000 quarantine and immigration officials, doctors and other health workers will be vaccinated by the end of this year, Health and Welfare Ministry official Kishiko Yamaguchi said.

Bird flu is relatively difficult for humans to catch, but health authorities fear it could mutate into a form that is easily spread among humans, which could cause a flu pandemic.

The Japanese health ministry is also considering vaccinating 10 million more people, including lawmakers, police, health workers and others in "lifeline" professions, if the initial tests prove safe and effective, Yamaguchi said.

World Health Organization spokesman Gregory Hartl called the planned vaccinations "a big roll of the dice," but said the WHO didn't oppose it.

"Obviously, the Japanese think there's some benefit to be had from this, and we are not going to prevent an individual country from using their resources," he said.

At least 239 people worldwide have died from bird flu since 2003, according to WHO. Most human cases have been linked to contact with infected poultry.