Japan lodged a formal protest via China's embassy against what it called an incursion by a Chinese survey ship into its territorial waters Saturday, the Japanese foreign ministry said.
The ministry expressed strong concern after the ship was spotted near Kagoshima Prefecture, southwestern Japan, early in the morning.
The Chinese ship, confirmed in territorial waters at 6 am local time, left shortly before 8 am, according to Japan's Defence Ministry, adding it was monitored by a Japanese military vessel and plane.
Recently, China's increasingly assertive activity around Japanese waters and airspace has caused unease among Japanese defence officials, also concerned about the growing military cooperation between the Chinese and Russian air forces.
This follows Tokyo's protest after a Chinese military aircraft briefly entered Japan'ssouthwestern airspace on Monday. It was the first time the Japanese Self Defence Force detected a Chinese military aircraft in Japan's airspace.
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