The Chinese and Russian militaries have sent a flotilla of 10 warships through the narrow Tsugaru Strait in northern Japan for the first time, in a signal of the two countries’ growing defense cooperation and an apparent response to a landmark nuclear submarine deal between the U.S., Britain and Australia.

The military vessels sailed through the narrow strait separating Japan’s main island and Hokkaido on Monday, the Defense Ministry said Tuesday.

It was the first time the ministry had confirmed a joint sailing by the two countries' militaries through the waterway, which connects the Sea of Japan and the Pacific Ocean.