Japan's Maritime Self-Defense Force says it conducted a joint drill with the US Navy in the South China Sea on Tuesday, sending a submarine to take part in such an exercise for the first time.
The MSDF announced that in addition to the submarine, two destroyers and a patrol plane took part in the drill. The US Navy sent a destroyer and patrol aircraft.
The exercise reportedly included anti-submarine training, which involved tracking a sub navigating underwater.
It's unusual for the whereabouts and activities of submarines to be disclosed, because their mission is undetected surveillance and patrol.
The MSDF says it is the first time Japan and the US have conducted a joint drill in the South China Sea with a submarine.
China has been stepping up its presence in the South China Sea by such means as building artificial islands to set up military bases.
The United States has been trying to counter such moves by creating a new security framework in the Indo-Pacific region, encompassing the South China Sea.
Observers say the joint drill is designed to underscore the Japan-US partnership and its aim of realizing a free and open Indo-Pacific, and also to keep China in check.