Japan has protested to China for allowing a gas drilling vessel to operate in disputed waters in the East China Sea, the government said on Friday. Japan and China agreed to jointly develop gas fields in the area in 2008 but talks have since stopped.
"It is extremely regrettable that China continues its unilateral development in the sea area in a situation where the maritime boundary between Japan and China has not been fixed in the East China Sea," Chief Cabinet Secretary Yoshihide Suga told a news conference.
Suga said Japan would continue to urge China to return to talks.
Relations between the two nations have improved in recent years after deteriorating sharply in 2012, when Japan nationalised a cluster of East China Sea islets that China also claims.
China's ties with Japan have also long been poisoned by what Beijing sees as Tokyo's failure to atone for its occupation of parts of China before and during World War Two.
(Reuters reporting by Kaori Kaneko; Editing by Nick Macfie)
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