Japan has accused China of unilaterally exploring gas deposits in the East China Sea, in violation of an agreement to jointly develop disputed areas.
Chief Cabinet Secretary Osamu Fujimura told reporters Wednesday that Japan protested to China after a flare was seen Tuesday at a Chinese structure at an undersea gas deposit. Japan has made similar complaints several times in the past.
"We have detected a flare, a sign that it is highly likely that there is a gas development going on," Fujimura said. "Any unilateral exploration is unacceptable."
The deposit, known as Kashi in Japan and Tianwaitian in China, sits near a median line of the two countries' overlapping exclusive economic zones.
Japan and China agreed in 2008 to suspend unilateral digging in that field while continuing talks, but talks have stalled since 2010, following a diplomatic spat stemming from a maritime collision near disputed southern islands claimed by both countries, as well as Taiwan.
Fujimura said China's activity around the disputed field violates the agreement.
A Foreign Ministry official in charge of China affairs said later Wednesday that Beijing justified its activity, repeating its claim that China has sovereignty over the area.
Days earlier, Beijing accused Tokyo of naming a group of uninhabited southern islands in nearby waters that both countries claim.
Four of the islands are in the disputed Senkaku, or Diaoyu, chain in the East China Sea. The islands, also claimed by China and Taiwan, have been a flashpoint in diplomatic relations.
Beijing said Tokyo's naming attempt is "illegal and invalid," the ministry official said on condition of anonymity because of sensitivity of the issue. Tokyo responded by saying the islands are an integral part of Japan.
Japan's Cabinet Office has said it will use the names for new maps, adding that the islets are within Japan's established exclusive economic zone and will not change any maritime boundaries.
The islands are surrounded by rich fishing grounds and are regularly occupied by nationalists from both sides.
Japan and China have often quarreled over the islands, the contested gas fields and lingering animosity over Japan's occupation of China in much of the first half of the 1900s.