"China's oil and gas exploration on its own continental shelf in the East China Sea is a normal exploration activity. It is beyond reproach," foreign ministry spokesman Qin Gang said in a statement on the ministry's website.

"China and Japan's dispute over the demarcation in the East Sea should be resolved through negotiation," the statement said.

"China is opposed to Japan making new conflicts and troubles."

The statement was referring to Japan's protest this week that state-owned China National Offshore Oil Corp had begun the first stage of gas production in the Chunxiao field.

Japan and China have long disputed their sea border and Beijing insists its exploration in the East China Sea is within its territorial waters.

Japan says the gas field lies too close to its exclusive economic zone -- a delineation not recognized by China -- and fears China might siphon off resources from an area Japan claims as its territory.

Tensions escalated between the two countries last year after Japan said it had spotted flames indicating China had started production close to the area where Japan claims development rights. China began test-drilling in 2003.

The two countries have so far failed to find common ground during many rounds of talks.