A Maritime Self-Defense Force submarine has conducted joint military drills with the U.S. Navy in the South China Sea for the first time — a move analysts say was intended to highlight the two navies' growing capabilities amid China's moves in the region.

The MSDF said in a statement released late Tuesday that the exercise conducted earlier in the day focused on anti-submarine warfare (ASW) and included one of Japan’s largest warships, the Kaga helicopter carrier, as well as a destroyer and a patrol plane, together with a U.S. destroyer and patrol aircraft, in the first such joint drills in the South China Sea.

The announcement of the submarine’s location was unusual as both the Japanese and U.S. rarely disclose the whereabouts of their so-called silent service. Japanese submarines are among the world’s most modern and sophisticated, observers say, and their MSDF operators among the best trained.