TOKYO—Japan has detected what it believes to be a Chinese submarine off a southern Japanese island, the defense ministry said Sunday, heightening Japan’s alert levels in the East China Sea as the Chinese communist regime increases its military activities. The submarine remained submerged, but the ministry said in a statement that it believes the submarine is Chinese because a Chinese Luyang III-class guided missile destroyer is near the submarine. The submarine moved northwest off the eastern coast of Amami Ōshima Island, about 420 miles northeast of the disputed East China Sea islands controlled by Japan but also claimed by Beijing, the ministry said. The submarine on Sunday morning was heading west in the East China Sea. Neither the submarine, nor the ship entered Japanese territorial waters. Under international law, submarines passing off the coast of another country are required to surface and show a national flag inside territorial waters. Japan’s …