Japan's Defense Ministry says Chinese and Russian naval ships jointly sailed between two islands of Okinawa Prefecture. This is the first confirmation of its kind. But the vessels did not violate Japan's territorial waters.

The ministry disclosed that on Thursday, six Chinese and five Russian vessels traversed between Okinawa's main island and Miyakojima Island in southwestern Japan before proceeding to the East China Sea.

Earlier, on Tuesday, the Maritime Self-Defense Force had detected the ships navigating westward in waters off Japan's southernmost Pacific island of Okinotorishima.

The ministry says the vessels are believed to be engaging in a joint patrol that the Russian defense ministry announced late last month had begun in the Pacific.

Japan's Defense Ministry expressed grave concerns over security and remains on alert while monitoring the situation.

In September last year, the ministry confirmed similar joint navigations by Chinese and Russian navy ships in waters near Japan.