Japan's defense ministry has announced that it believes a Chinese submarine is in waters near its southern islands, saying the submarine was spotted Sunday traversing to the northwest just outside Japanese territorial waters near Amami Oshima island.
During the same sighting a Chinese destroyer was also spotted in the area. It's unclear whether the submarine ever breached Japan's waters - based on official statements it doesn't appear so - and the incident comes after years of Tokyo complaining of such aggressive Chinese incursions.
According to Reuters, the weekend announcement indicated Japan's Maritime Self-Defense Force "identified the vessels in a contiguous zone, which is outside territorial waters where vessels are required to identify themselves." Amami Oshima Island is far south off the Japanese mainland.
Japan's Defense Minister Nobuo Kishi ordered his staff "gather information and maintain vigilant surveillance with a sense of urgency," according to the statement.
The Japanese military response included sending three reconnaissance aircraft and two destroyers to the area in order to assess and observe the Chinese vessel's intentions.
After the sighting, and as Japan continued to monitor the vessel, "The submarine continued underwater westward in the ocean near Yokoate Island," the defense ministry said. It's said to now be heading west into the East China Sea, near islands currently controlled by Japan but claimed by China.
International maritime law and norms requires that any submarine passing off a foreign county's coast must surface and identify itself by flying a national flag should it come to within that country's territorial waters.
In recent years such hostile Japan-China encounters, particularly around disputed islands claimed by both, have become increasing occurrences, also as Tokyo has lately more openly declared itself in Washington's corner when to comes to the intensifying Taiwan status issue.