Japan has launched a protest against China's drilling of gas in disputed waters in the East China Sea and called for a resumption of talks over the disputed area.
"Japan said it deeply regrets China's one-sided decision to start extracting natural gas from the region, and calls on China to stop its activities," a Foreign Ministry official said on condition of anonymity, citing government policy.
Tuesday's protest came after Tokyo said it had confirmed that China was extracting natural gas from the Tianwaitian oil field in the East China Sea, which separates China's eastern coast and Japan's southern island chain of Okinawa.
Tokyo and Beijing have been feuding over claims to the undersea gas deposits, amid a broader diplomatic row that has soured bilateral relations in recent months.
In the past, Japan has proposed that Tokyo and Beijing decide on a line between their economic zones and jointly exploit resources in the area. But talks that began earlier this year have been suspended since May.
China said Tuesday it was within its rights to continue with new gas drilling activity in the area, but was willing to negotiate with Japan over the territorial waters.
Under the U.N. Convention on the Law of the Sea, coastal countries can claim an economic zone extending 200 nautical miles (230 miles; 370 kilometers) from their shores. Both Japan and China signed the treaty, but the United Nations has until May 2009 to rule on the claims.
China also bases its claim on a separate international treaty that lets coastal countries extend their borders to the edge of the undersea continental shelf.
In July, Beijing formally protested Tokyo's decision to give private oil company Teikoku Oil Co. drilling rights in the region, calling it a severe provocation. Teikoku and several other Japanese oil companies first applied for drilling rights in the late 1960s following a U.N. report about possible rich undersea deposits.
But China made the first move, launching an exploration operation and construction of an undersea pipeline last year, the AP reports.