Japan has called on China to stop developing gas fields near a disputed maritime border in the East China Sea, as fresh talks began on the issue.
Japan and China cannot agree on where their sea borders should lie
Japan formally complained to China last week after it said a Chinese company's operations could take gas from the Japanese side of the border.
Both countries have suggested joint exploration of the area, but they are at odds about how to proceed.
The issue is one of the main irritants in testy relations between the two.
Japan's Foreign Minister Nobutaka Machimura said before the talks began in Tokyo that Japan would ask China for more information.
"We will ask them to stop gas development on their own, and stop drilling if they are going ahead with drilling, without giving us any information," he said.
China and Japan's exclusive economic zones (EEZs) overlap
Japan claims EEZ extends 200 nautical miles from its shore, while China claims EEZ extends to edge of its continental shelf
Two countries have never agreed a maritime border
The UN says it will arbitrate by May 2009
Also dispute ownership of Senkaku/Diaoyu islands
Tokyo is worried that Chinese operations could tap into reserves Japan claims as its own.
Earlier this month, the Japanese government complained when flames were seen in the Tianwaitian gas field, which is very close to the area Japan claims rights to.
In July, China protested against Japan's decision to give exploration rights to the Japanese company Teikoku Oil - the first time Japan had taken such an active step to exploit the area's reserves.
Japan and China, both seeking new reserves of oil and gas for their energy-hungry economies, have frequently clashed over drilling rights in the region due to a long-standing disagreement over their sea borders.
According to a UN convention, both countries can claim an exclusive economic zone (EEZ) extending 200 nautical miles (370 km) from their shores.
But China claims its EEZ on the basis of its continental shelf, which extends into Japan's claimed area.
The two nations have also clashed over other issues this year.
In April Tokyo approved a set of controversial history textbooks, which critics say whitewashed its record during World War II.
The move triggered rare public rallies in China - which analysts say had Beijing's tacit approval. Angry Chinese protesters marched in several major cities and targeted Japanese buildings.
Further disputes followed, over Japan's quest to gain a permanent seat at the UN Security Council, as well as ongoing compensation requests for Chinese survivors of Japanese atrocities during the war.