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 disputed waters, less than two months after it positioned a giant drilling platform in territory claimed by Vietnam.

Coordinates posted on the website of China's Maritime Safety Administration showed that two rigs had been deployed roughly between southern China and the Pratas islands, which are occupied by Taiwan. A further platform was towed close to the Chinese coast. Earlier this week, it gave coordinates for a fourth rig, which it said would be positioned just outside Vietnam's exclusive economic zone by Friday.

The action 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 numerous islands and reefs is in dispute.

Wang Ching-hsiu, deputy director of Taiwan's Land Administration Department, said Taipei claimed an exclusive economic zone around the Pratas Islands, but he declined to comment on the rig deployments.

China, which regards Taiwan as a renegade province and claims about 90 percent of the South China Sea, said the rigs were in waters close to Guangdong province and Hainan island. The Philippines, Vietnam, Malaysia, Brunei and Taiwan also have claims to parts of the sea.

"For these normal activities there is no need for over-reading or to make any particular links," Chinese Foreign Ministry spokeswoman Hua Chunying told a daily briefing in Beijing. "Please don't worry, there won't be any problem."

The U.S. State Department said it would withhold judgment because it not have enough information about the placement of the rigs. But Washington repeated its long-standing view that it would be troubled if they were in disputed waters.

"If a rig were placed in disputed waters, that would be a concern," State Department spokeswoman Jen Psaki told reporters in Washington. "We certainly have a national interest in the maintenance of peace and stability in the region."

All four rigs are listed as being operated by China Oilfield Services Ltd (COSL), the oil service arm of state-run China National Offshore Oil Company (CNOOC) Group, according to COSL's 2013 annual report.

Three are deepwater platforms and one is a jackup rig used in shallow water.

Reached by telephone, COSL Chief Executive Li Yong said he had seen the maritime announcement but declined to comment. A CNOOC spokesman also declined to comment.

The Global Times, a popular tabloid published by the Communist Party's official People's Daily, quoted Zhuang Guotu, director of the Center for Southeast Asian Studies at Xiamen University, as calling the rig deployment a "strategic move."

"The increase in oil rigs will inevitably jab a sensitive nerve for Vietnam and the Philippines," Zhuang said.

Al Jazeera and Reuters