"Five of the nine ballistic missiles launched by China are believed to have landed within Japan's EEZ," Nobuo Kishi told reporters, as China holds massive military drills in the waters around Taiwan.

Japan had "lodged a protest with China through diplomatic channels", Kishi said, calling the matter "a serious problem that affects our national security and the safety of our citizens".

Parts of Japan's southernmost island region Okinawa are close to Taiwan. Kishi said it was the first time Chinese ballistic missiles had landed in Japan's exclusive economic zone (EEZ).

The EEZ extends up to 200 nautical miles from Japan's coastline, beyond the limits of its territorial waters.

The figure of nine missiles fired was an assessment by the Japanese side, Kishi said.

China is holding its largest-ever military exercises around Taiwan, which it considers its territory and has vowed to one day seize, by force if necessary.

The show of force was sparked by US House Speaker Nancy Pelosi's visit to the self-ruled island, despite stern warnings from Beijing.

The drills began around 12 pm (0400 GMT), and involved a "conventional missile firepower assault" in waters to the east of Taiwan, the Chinese military said.