Japan on Monday started vaccinating medical workers against swine flu, which is known to have killed 27 people in the country, the health ministry said.

The country has a limited supply of the domestically-made vaccine, with doses for about 1.18 million people available in the first batch as Japan heads into the autumn-winter flu season, a ministry official said.

"Pregnant women and those with chronic diseases are the next priority group and will start getting the vaccination from early November," the official said. "Babies and small children will follow, starting in late December."

But as of Monday, only 23 prefectures began the vaccinations while 23 other prefectures plan to kick off the programme later this week, with Tokyo starting some time after October 26, the ministry said.

"We have a lot more doctors and hospitals in Tokyo, which means we need more time for preparation," a Tokyo official said.

In western Osaka prefecture, nearly 180,000 medical workers are waiting for the shot, with local officials worried about a supply shortage.

The A(H1N1) virus is believed to have first entered Japan in May, and government officials fear the pandemic will spread further in the coming influenza season.