Five Chinese naval ships entered the Sea of Japan heading in the direction of Russia over the weekend, Tokyo said after Beijing announced joint military drills with Moscow.
The Japanese defence ministry "confirmed that these vessels sailed north-eastwards through the Tsushima Strait towards the Sea of Japan from Saturday to Sunday", it said in a press release on Monday.
Japan's Self-Defence Forces "conducted vigilance monitoring and intelligence gathering" with a vessel and patrol plane, the ministry said, releasing photos of the ships.
It did not say where the Chinese ships were heading but local media reports suggested that they were on their way to the drills with Russia announced by Beijing on Monday.
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 Japanese territorial waters.
Beijing gave no date for the "North-Joint 2024" exercises in the skies and around the Sea of Japan and Sea of Okhotsk, off Russia's coast, saying only that they would take place this month.
China's growing economic and military clout and its assertiveness in territorial disputes, most recently with the Philippines, has rattled the United States and its allies.
Russia and China have also ramped up military and economic cooperation in recent years, with both railing against "Western hegemony", particularly what they see as US domination of global affairs.
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, Japan protested after a Chinese naval ship entered its territorial waters.
Japan and Russia also have a territorial dispute over the Kuril Islands -- known in Japan as the Northern Territories -- and their relations have deteriorated sharply since the start of the Ukraine war.
Japanese and Chinese vessels have been involved in tense incidents in other areas, in particular, the remote Senkaku islands in the East China Sea claimed by Beijing, which calls them the Diaoyus.
Tokyo has reported the presence of Chinese coastguard vessels, a naval ship and even a nuclear-powered submarine in the area, and there have been a series of confrontations between Japanese coastguard vessels and Chinese fishing boats.
Last month Japan's defence ministry sought 8.5 trillion yen ($59 billion) for the next fiscal year, its largest ever initial budget request, as part of the country's five-year, 43-trillion-yen defence buildup plan through March 2028.