China’s Ministry of National Defense has announced it will hold joint naval and air drills with Russia starting this month, underlining the close ties between the militaries of the two countries.

The ministry said the “Northern United-2024” exercises would take place in the Sea of Japan and the Sea of Okhotsk farther north, but gave no date or further details.

The Japanese Ministry of Defense said it had observed five Chinese navy vessels “sailing north-eastwards through the Tsushima Strait towards the Sea of Japan from Saturday to Sunday”.

The Japan Self-Defense Forces “conducted vigilance monitoring and intelligence gathering” with a vessel and patrol plane, the ministry said in a statement on Monday, releasing photos of the ships.

The Tsushima Strait lies between South Korea and Japan and connects the South China Sea and the Sea of Japan, known as the East Sea to Koreans, and is not within Japan’s territorial waters.

Announcing the drills, China said they were designed to improve strategic cooperation between the two countries and “strengthen their ability to jointly deal with security threats”.

The notice also said the two navies would cruise together in the Pacific, the fifth time they have done so, and together take part in Russia’s “Great Ocean-24” exercise. No further details were provided.

China has deepened its relationship with Russia since Moscow launched its full-scale invasion of Ukraine in February 2022. It has not condemned the invasion but has called regularly for an end to the fighting.

It has also emerged as a crucial economic lifeline for Russia, buying its oil and gas and supplying electronics and other items.

China’s growing economic and military clout and its assertiveness in territorial disputes, most recently with the Philippines over the South China Sea, has rattled the United States and its allies including Japan.

On August 26, Japan scrambled fighter jets after what it called the first confirmed incursion by a Chinese military aircraft into its airspace.

Last week, it lodged a protest after a Chinese naval ship entered its territorial waters.

Japan has territorial disputes with China – over the remote Senkaku islands in the East China Sea – and with Russia – over the Kuril Islands between Hokkaido and Kamchatka.

Relations between Tokyo and Moscow have deteriorated sharply since the Ukrainian invasion, with Japan joining the United States and other Kyiv allies to join sanctions against Moscow.