Foreign Minister Fumio Kishida said Tuesday said Japan had lodged a protest with China over its apparent deployment of drilling rigs near the median line separating the two countries’ economic zones in the East China Sea.

Japan also urged China to swiftly resume stalled negotiations to cooperate over oil and gas resources in the area. Those talks began in 2008 but broke down two years later amid rising tensions.

Both countries claim the East China Sea islands which are controlled by Japan. Tokyo says China has built oil and gas extracting structures on its side of the median line, which may siphon off resources from beneath the Japanese side.

A Japanese government official said Tuesday China’s mobile drilling ships had been spotted in the region, and were believed to have been boring for gas.

It was "extremely regrettable that China is unilaterally continuing its development activity" near the median line, Japan's top government spokesperson Yoshihide Suga told reporters.

China's Foreign Ministry on Wednesday dismissed the remarks, saying “the so-called issue of 'unilateral exploitation' does not exist."

"China's oil and gas activities in the East China Sea are all located in maritime areas indisputably under Chinese jurisdiction," it was quoted as saying.

The gas field reportedly lies in an area where both countries' exclusive economic zones EEZs overlap but they bitterly disagree on how it should be demarcated.

While Japan says the median line between the two nations should mark the limits of their respective EEZs, China insists the border should be drawn closer to Japan, taking into account the continental shelf and other features of the ocean.

On Wednesday, China's Foreign Ministry said it rejected the idea of a median line, calling it "Japan's unilateral proposition."

With tensions rising in the East China Sea, a seafood-packed body of water through which nearly $5 trillion in trade flows each year, the US has waded into the dispute.   

President Donald Trump pledged earlier this year that the US would rally behind Japan in a possible serious showdown with China.