Japan protests Russian warships passing through nearby strait

March 15 (UPI) -- Japan lodged a complaint through diplomatic channels after a group of Russian navy ships passed through a strait near Hokkaido while conducting exercises related to the invasion of Ukraine, Japanese officials said Tuesday.

A group of six Russian navy ships passed between Japan's northernmost prefecture and Russia's Sakhalin Island on Monday after participating in military drills, Japan's defense ministry said Tuesday.

Tokyo complained to Moscow and is watching Russia's increasing military activities near Japan with grave concern, top government spokesman Hirokazu Matsuno said at a press conference.

"The Russian military has been carrying out extraordinary naval drills on a large scale in the Sea of Okhotsk and other areas since February in conjunction with Moscow's invasion of Ukraine, and the six ships are believed to have participated in the drills," Matsuno said, according to a translation by news agency Jiji Press.

The Russian ships were spotted in the early hours of Monday passing through the Soya Strait about 80 miles southeast of Cape Soya on Hokkaido, Japan's Joint Staff said in a statement.

Among the vessels were a Udaloy-class destroyer and three Kilo-class submarines, the statement said.

The incident is the latest in a series of Russian military drills near Japan that have raised alarms in Tokyo.

On Thursday and Friday, a total of 10 Russian warships passed through the Tsugaru Strait between Hokkaido and the main Japanese island of Honshu, Japan's military said.

Russia also conducted a missile drill on a disputed island near Hokkaido last Thursday, prompting another diplomatic complaint, The Yomiuri Shimbun reported. Earlier in the month, Japan claimed a Russian helicopter violated its airspace and scrambled a fighter jet in response.

Japan has joined Western nations in imposing a series of punishing sanctions on Russian banks and oligarchs over the invasion of Ukraine and on Tuesday announced that it would freeze the assets of an additional 17 individuals, including billionaire Viktor Vekselberg.

Tokyo has also committed to providing $200 million in aid to Ukraine and sent bulletproof vests and helmets to the country in a rare move that required a revision of its guidelines on the transfer of defense equipment.