Japan has lodged an official protest with China after discovering what it claimed were efforts by Beijing to develop gas fields in disputed waters in the East China Sea.
That agreement saw Japan and China agree to jointly develop undersea gas reserves in the disputed area, with a ban on independent drilling by either country.
But China insists the border should be drawn closer to Japan, taking into account the continental shelf and other features of the ocean.
The two countries are embroiled in a separate row over disputed islands elsewhere in the East China Sea.
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