"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 "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".

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

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, adding that the five appeared to have landed southwest of Okinawa's Hateruma island.

The drills began on Thursday, and involved a "conventional missile firepower assault" in waters to the east of Taiwan, the Chinese military said.

Japan had on Wednesday expressed concern to China over the drills, saying they were planned to take place in maritime areas which overlap with its EEZ.

All nine missiles were believed to have fallen within areas designated by China for the military exercises, Kishi said.

The minister declined to comment on China's intentions regarding the drills, but nonetheless called them "extremely menacing".