Japan has lodged a protest with China after discovering what it says were efforts by Beijing to develop gas fields in disputed waters of the East China Sea.
Tokyo's foreign ministry said late Monday it had confirmed that Beijing was setting up drilling rigs in the area -- where the two countries' exclusive economic zones (EEZ) claims overlap -- and submitted a complaint to the Chinese embassy.
"It is extremely regrettable that China is advancing unilateral development," the ministry said, noting it had taken place on the Chinese side of the de facto maritime border.
The ministry accused China of positioning 21 suspected drilling rigs, with Tokyo fearing gas on the Japanese side could also be extracted.
Japan "issued a strong protest" to the Chinese embassy, the ministry said.
It "strongly urged China for an early resumption of talks on the implementation" of a 2008 bilateral agreement regarding the development of resources in the East China Sea, it added.
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 negotiations over how to implement the deal were suspended in 2010.
Japan has long insisted the median line between the two nations should mark the limits of their respective EEZs.
China, however, insists the border should be drawn closer to Japan, taking into account the continental shelf and other ocean features.
The two countries are embroiled in a separate row over disputed islands elsewhere in the East China Sea.
China claims the string of islands -- which Japan refers to as the Senkakus and are known as the Diaoyu by Beijing -- as its own, and regularly sends ships and aircraft into the area to test Tokyo's response times.
China also has disputes with several other nations in the South China Sea, which it claims in its entirety.
