Japan's Defence Ministry said on Sunday that a submarine sighted near its southern islands is believed to be Chinese.
According to the ministry, the submarine continued sailing westward in the ocean near Yokoate Island. In turn, Japanese Defence Minister Nobuo Kishi instructed his staff to "gather information and maintain vigilant surveillance with a sense of urgency".
On Friday, Japan's navy spotted a submerged vessel sailing northwest near Amami Oshima island, part of Kagoshima prefecture, According to the Japanese Defence Ministry, a Chinese destroyer was also seen in the vicinity.
In recent years, Tokyo has repeatedly voiced concerns over Beijing's activities in the region, especially with regard to the disputed Senkaku Islands, known in China as the Diaoyudao Islands. Tokyo claims to have had sovereignty over the islands since 1895, while Beijing argues that they were marked as Chinese territory on Japanese maps dated 1783 and 1785.
China claims that the islands were illegally seized by Japan. Tokyo, for its part, suggests that China only became interested in the islands in the 1970s because the nearby territory was found to potentially hold oil reserves.