Japan has lodged a new protest against China's drilling for gas near a disputed part of the East China Sea.
Japan and China are increasingly competing for resources
Trade Minister Shoichi Nakagawa said he had information that a Chinese consortium was laying pipes to prepare to tap the area's huge gas resources.
"We have protested to the Chinese government through diplomatic channels but we have not received any satisfactory reply," he said.
The move comes at a sensitive time for already strained Japan-China relations.
Countries in the region are currently preparing to mark the 60th anniversary of the end of World War II on 15 August, and Chinese-Japanese relations have been hurt by a row over history textbooks.
The gas dispute stems from a long-standing disagreement over the extent of both Japan and China's claims to gas-rich areas under the East China Sea.
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 gas fields where China has been test drilling - known as the Chunxiao field - lie in waters which both sides agree belong to China.
But they are very close to Japan's claimed EEZ, and Japan is now worried that China is preparing to siphon off gas buried under the seabed on the Japanese side.
"We found that possibilities are high that the Chinese side has placed a pipeline, and is ready to put it to practical use at any time," Mr Nakagawa told reporters on Wednesday.
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
The latest dispute comes a month after 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 fuel reserves.
Teikoku Oil asked for the rights - on Japan's side of its claimed EEZ - in April, after Tokyo signalled a change in policy to allow test drilling.
China began test drilling in 2003.
The two nations have also clashed over other issues this year, and tensions between them are already strained.
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.
Japan has just marked 60 years since Hiroshima and Nagasaki
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.