China has sent four more oil rigs into the South China Sea in a sign that Beijing is stepping up its exploration for oil and gas in the tense region, less than two months after it positioned a giant drilling platform in waters claimed by Vietnam.

The announcement comes at a time when many countries in Asia are nervous at Beijing's increasing assertiveness in the potentially energy-rich waters, where sovereignty over countless islands and reefs is in dispute.

Coordinates posted on the website of China's Maritime Safety Administration showed the Nanhai No. 2 and five rigs had been deployed roughly between China's southern Guangdong province and the Pratas Islands, which are occupied by Taiwan. The Nanhai 4 rig was towed to waters close to the Chinese coast.