China Coast Guard Vessel
A Chinese Coast Guard vessel (R) passes near the Chinese oil rig, Haiyang Shi You 981 (L) in the South China Sea, about 210 km (130 miles) from the coast of Vietnam June 13, 2014. Reuters/Nguyen Minh

(Reuters) - China has sent four 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.

Coordinates posted on the website of China's Maritime Safety Administration showed the Nanhai number 2 and 5 rigs had been deployed roughly between southern China and the Pratas islands, which are occupied by Taiwan. The Nanhai 4 rig was towed close to the Chinese coast.

The agency did not say who owns the rigs.

Earlier this week, it gave coordinates for a fourth rig, the Nanhai 9, which it said would be positioned just outside Vietnam's exclusive economic zone by Friday.

The announcement comes at a time when many countries in Asia, particularly Vietnam and the Philippines, are nervous at China's increasing assertiveness in the South China Sea, where sovereignty over countless islands and reefs is in dispute.

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.

China's state oil behemoth CNOOC Ltd has said it had four new projects scheduled to come on stream in the western and eastern South China Sea in the second half of 2014.

It was unclear if the four rigs were part of those projects. A CNOOC spokesman declined to comment, but the company has long said that in a bid to boost production it wanted to explore in deeper waters off China.

CNOOC has said it would increase by up to a third its annual capital spending for 2014 to almost $20 billion.

Anti-Chinese violence flared in Vietnam last month after a $1 billion deepwater rig owned by CNOOC Group, the parent of the listed unit, was parked 240 km (150 miles) off the coast of Vietnam.

Hanoi says the rig is in its 200-nautical mile exclusive economic zone and on its continental shelf. China has said the rig was operating completely within its waters.

China claims about 90 percent of the potentially energy-rich South China Sea. The Philippines, Vietnam, Malaysia, Brunei and Taiwan also have claims to parts of the waters.

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