Three Chinese maritime surveillance ships entered Japanese territorial waters Monday near the Senkaku Islands, the Japan Coast Guard said.
In a separate incident near the area, an unidentified submarine was spotted by a P-3C Maritime Self-Defense Force patrol aircraft sailing in Japan's contiguous zone south of Okinawa's Kume Island at midnight Sunday. A Defense Ministry official said the vessel was probably a Chinese submarine.
The submarine left the contiguous zone around Kume on Monday morning, the ministry said.
Sailing in Japan's contiguous zone — a stretch of water outside its territorial seas where Japanese law may be applied — poses no problem in terms of international law but the ministry is stepping up its guard.
The latest intrusions by Chinese vessels into Japanese waters prompted Shinsuke Sugiyama, director general of the Foreign Ministry's Asian and Oceanian Affairs Bureau, to telephone Han Zhiqiang, China's minister to Japan, to lodge a protest, a ministry official said.
The three ships — the Haijian 15, 50 and 66 — entered Japanese territorial waters in the East China Sea around 9 a.m. in succession from north of Kubajima in the Senkaku group, according to the 11th Regional Coast Guard Headquarters in Naha.