The Japanese Maritime Self-Defense Force is monitoring a flurry of Russian and Chinese naval activities in waters off Japan.

MSDF officials say one of their vessels spotted seven Russian navy ships passing through the Izu Island chain in the Pacific south of Tokyo from Thursday to Friday.

They say a helicopter was seen taking off from a frigate. The fleet was earlier observed off Chiba Prefecture neighboring Tokyo.

Defense Ministry officials say nine other Russian naval vessels were seen about 40 kilometers off Hokkaido in the north on Friday morning.

They say the fleet passed the Soya Strait and entered the Sea of Japan.

Officials are trying to determine whether the activities are related to a large-scale drill Russia plans to stage in the Pacific later this month.

Defense officials also say two Chinese destroyers were spotted passing the Soya Strait toward the Sea of Okhotsk on Thursday.

The vessels were part of a fleet of four warships sailing in the Sea of Japan. The other two reached the Pacific on Thursday through the Tsugaru Strait.

Both the Soya and Tsugaru straits are international waterways where foreign vessels, including warships, are allowed to navigate.