Japan says China has shipped drilling equipment to a disputed gas field in the East China Sea, fuelling diplomatic tensions between the Asian giants.
China has previously refrained from unilateral drilling at Japan's request.
Both Beijing and Tokyo claim exploration rights over the area.
China scrapped scheduled talks with Japan over joint exploration of the field last week after Japan arrested the captain of a Chinese trawler near disputed islands in the East China Sea.
All have been released except the captain who remains in detention.
The Chinese fishing boat reportedly rammed Japanese coastguard patrol boats which had been trying to intercept it.
Prosecutors have until Sunday to charge or release the captain.
There are unconfirmed reports of anti-Japanese demonstrations in China including Shanghai, Nanjing and Guangzhou this weekend.
Security around the Japanese Embassy in Beijing has been stepped up.
Japan's foreign ministry says the drilling equipment has been moved to a platform above a natural gas field, known as Chunxiao in China and Shirakaba in Japan.
Aerial photos taken by surveillance missions by Japan forces showed the shipments and workers engaged in what appeared to be preparations for drilling, Kyodo News reported.
But Japan's Foreign Minister Katsuya Okada, who stepped down on Friday, said Beijing had told Tokyo that the equipment was for "repair work".
He said he "strongly expects" that China would refrain from starting gas extraction.
China started drilling in Chunxiao in 2003, inflaming tensions with Japan, which expressed fears that Beijing could siphon gas from what it considered its own side.
China contends that the gas field falls easily within its maritime zone, but Japan contests this.
The BBC's Michael Bristow in Beijing says it is perhaps a small development - but comes at a time when relations between the two countries are as bad as they have been for several years.
China has made repeated protests over the detention of the Chinese fishing boat and its captain near the uninhabited islands known as Diaoyu in China and Senkaku in Japan, and warned that it could harm bilateral ties.
Ties between China and Japan have improved in recent years - these two nations are now major trading partners.
But this latest incident is a reminder that there are many unresolved issues between the two countries - issues that have the potential to develop into major disputes, our correspondent says.