A group of 10 naval vessels from China and Russia sailed through a strait separating Japan's main island and its northern island of Hokkaido on Monday, the Japanese government said, adding that it is closely watching such activities.

It was the first time Japan has confirmed the passage of Chinese and Russian naval vessels sailing together through the Tsugaru Strait, which separates the Sea of Japan from the Pacific.

While the strait is regarded as international waters, Japan's ties with...