Japan has expressed serious concern to China over the construction of a drilling rig near a disputed gas field in the East China Sea.
Chief Cabinet Secretary Yoshihide Suga said Tokyo "cannot accept" China's development near the area.
The construction is in China's area, but Japan is concerned China would be able to siphon gas from its side.
A co-operation agreement between the countries had broken down amid confrontation over disputed territory.
The economic zones of both countries overlap at the gas field which the two countries claim exclusive excavation rights to.
They had agreed in 2008 that Japanese companies would invest in the area, but the agreement eventually stalled.
Tokyo now says a vessel has been spotted drilling a platform on the Chinese side about 26km (16 miles) from the disputed gas field.
"Our position remains that we cannot accept China's unilateral development in this region where Japan's and China's claims overlap, while delimitation in East China Sea remains undefined," Mr Suga said.
"We have conveyed to China our serious concern about the activities of the crane vessel. We told them, through diplomatic channels, we cannot accept it," he said.
China has yet to respond to Japan's concerns.
The two countries have been locked in an ongoing row over disputed territories and waters, especially over islands known as Senkaku in Japan and Diaoyu in China.
Japan controls the islands and sparked anger in China when it bought three of them from their private Japanese owner late last year.