State-run CNOOC, China's top offshore oil producer, has issued a tender to invite foreign firms to bid for oil and gas blocks in the East and South China Sea, according to a statement on the company website.

CNOOC said it would offer 25 offshore blocks for foreign participation this year, including 17 in the South China Sea, three in the East China Sea and five others in the Yellow and Bohai Sea.

A CNOOC official in charge of the tender said none of the blocks offered are located in disputed territorial waters.

Relations between China and Japan, the world's second- and third-largest economies, have been troubled in recent years by a row over tiny, uninhabited islands in the East China Sea known as the Senkaku in Japan and the Diaoyu in China.

In the South China Sea, Vietnam, the Philippines, Taiwan, Malaysia, Brunei and China are involved in long-standing sovereignty disputes over the potentially oil- and gas-rich island chain.

Apart from diplomatic offensives and the flexing of military muscle, China has begun to assert its claims in the South China Sea by launching tenders of oil and gas blocks in disputed areas.

CNOOC in June invited foreign companies to jointly develop nine blocks in the western part of the South China Sea, a move Vietnam said was illegal because the blocks overlap its territorial waters and ownership is disputed.

The firm is also in a $5 billion drive to develop seven new gas fields in the East China Sea, possibly siphoning gas from the seabed beneath waters claimed by Japan.

CNOOC Chairman Wang Yilin has said the South China Sea forms a key part of achieving the company's ambitious goal to double its oil and gas production by 2020 and triple it by 2030 from 2010 levels.