TOKYO (REUTERS) - Japan has protested to China over signs it is pressing ahead with maritime gas exploration in the East China Sea despite Tokyo's repeated requests to stop, according to Japan's top government spokesman.

The exploration platforms are on the Chinese side of the median line between the two countries, but Japan accuses China of ignoring a 2008 agreement to maintain cooperation on resources development in an area where no official border has been drawn.

China said in July it had every right to drill in the East China Sea close to waters disputed with Japan, adding that it did not recognise the "unilateral" Japanese median line setting a boundary between the two.

Ties between China and Japan, the world's second- and third-largest economies, have already been strained by their conflicting claims over a group of tiny East China Sea islets and the legacy of Japan's wartime aggression.

"Earlier this month, flares were newly witnessed at two of the gas exploration platforms China had installed in the East China Sea," Chief Cabinet Secretary Yoshihide Suga told reporters. "It is extremely regrettable that China, despite our multiple representations, is carrying on with unilateral development in an area where no maritime border has been set. We protested to China through diplomatic channels right away."

Chinese Foreign Ministry spokesman Geng Shuang said on Thursday (Oct 13) Chinese gas exploration was carried out in waters"indisputably under Chinese jurisdiction".

"It is a matter completely within China's rights and jurisdiction," he told a daily news briefing in Beijing.

Japan is also at odds with China's South China Sea claims.

China claims most of the South China Sea, through which more than US$5 trillion (S$6.9 trillion) of trade moves annually. Brunei, Malaysia, the Philippines, Taiwan and Vietnam have rival claims.

Japan has no territorial claims over the waters, but much of the trade is to and from Japanese ports.