Japan's Defense Ministry says seven vessels belonging to the Russian and Chinese navies have entered the East China Sea after passing through a strait in southwestern Japan. Officials say the vessels appeared to be conducting a joint patrol.

Officials say three Chinese vessels sailed in the Pacific Ocean near the Izu Islands on Monday. They say four Russian vessels also cruised near the islands on Tuesday.

The ministry says the Japanese Maritime Self-Defense Force confirmed that the Russian and Chinese vessels joined up in waters south of Shikoku, one of Japan's four main islands.

Officials say the ships then moved through the Osumi Strait on Wednesday and Thursday, and traveled in two columns after entering the East China Sea.

This is the second time that Russian and Chinese naval vessels have passed through the strait together. The previous time was last October.

Russia's defense ministry announced last week that Russian and Chinese naval ships had been holding joint patrols in the Pacific Ocean.

Japanese officials say the vessels that sailed through the Osumi Strait are believed to be engaged in those patrols.

The chief of Japan's SDF Joint Staff, General Yamazaki Koji, told reporters that he thinks the Chinese and Russian navies are cooperating more closely than before.

He added that the SDF will stay vigilant and keep a close watch on developments.