May 8 (UPI) -- Japan is on guard after boats belonging to the Chinese coast guard entered Japanese territorial waters near Okinawa and the Senkaku Islands.
NHK and Kyodo news agency reported Monday four boats sailed into Japan-claimed waters in the morning.
Both Japan and China claim the Senkaku Islands, also known as the Diaoyu Islands in China.
It is the first time in two weeks Chinese vessels were seen in the disputed waters, according to reports.
Chinese vessels were also seen sailing near Uotsuri-Shima, the largest of the islands in late March.
Those boats, also traveling in a group of four, remained in Japan-claimed waters for about an hour and 30 minutes.
The Chinese vessels then headed outside Tokyo's territorial waters, about 13 to 27 miles from the area.
In previous cases, the Japanese foreign ministry filed complaints with the Chinese embassy in Tokyo, and the Japanese embassy in Beijing expressed concerns to the Chinese government.
The United States has said it would defend the uninhabited islands from foreign attacks.
In 2016, more than 100 Chinese ships trespassed into Japan's territorial waters, the second-largest annual number of Chinese ships entering disputed areas since Japan announced the nationalization of the Senkakus in September 2012.
China, Japan and Taiwan all claim the islands.