The U.S. Navy conducted a so-called freedom of navigation operation in southwestern Japan waters in December, in a move that clashes with Tokyo's claims of sovereignty.

According to the U.S. 7th Fleet, the Dec. 15 operation involved the navy's Alan Shepard supply vessel sailing in the vicinity of the Tsushima Strait between Kyushu, one of Japan's main islands, and South Korea. Its shipboard helicopter also took part.

The move is seen as emphasizing the U.S. stance of abiding by the international rules-based order — even when it conflicts with claims by its ally Japan — in a veiled message to China, which has become increasingly assertive in the East and South China seas.