Japan expressed opposition Wednesday to exploitation by China of a possible natural gas field that may stretch into the seabed under contested waters in the East China Sea.
"We conveyed our serious concerns (to the Chinese side) through diplomatic channels," Chief Cabinet Secretary Yoshihide Suga told a news conference.
In the latest development threatening to worsen bilateral relations that are already tense over a group of islands in the sea, Japan has confirmed that a large Chinese crane ship is building a mining facility at a point near the Japan-claimed border between their exclusive economic zones.
Although the point is in the Chinese EEZ, it is only 26 kilometers off the border, raising the prospect that the undersea gas field may extend into the Japanese side. Tokyo has proposed joint development of the area, but bilateral talks on the matter have stalled.
"It is not acceptable if China unilaterally develops (gas fields) in the waters where Japanese and Chinese claims overlap," the top government spokesman said.
In Beijing, Chinese Foreign Ministry spokeswoman Hua Chunying told a press conference that "China does not accept the Japanese protest."
Hua said China's exploration activity is "totally justifiable" as it is taking place "in waters under its own jurisdiction," noting also that Beijing has never accepted the so-called median line in the sea "unilaterally" claimed by Tokyo.
The issue stems from unsettled demarcation of the East China Sea where the two countries' economic waters overlap.
Under international law, coastal nations can claim an EEZ extending 200 nautical miles from their shores, with exclusive rights to exploration and use of marine resources.
But the waters between Japan and China are less than 400 nautical miles wide.
So Japan has set a so-called median line in the waters, equidistant from the two countries, as the EEZ demarcation and recognized the line's east side as Japan's EEZ under domestic law.
The area where China has started building the facility is within Japan's 200-nautical-mile zone.
Japan had previously confirmed China's gas development at four other points near the median line between their coastlines. China says its EEZ extends much further from the mainland, as far as waters around Okinawa.