China sent patrol boats to the South China Sea over the weekend to boost its surveillance of the area, heightening tensions with Vietnam following the launch of an offshore bid round by China National Offshore Oil Corp. that Hanoi claims is illegal.

Xinhua news agency reported Sunday that a patrol team of four China Marine Surveillance ships was dispatched to the Huayang reef in China's Nansha islands in the South China Sea to conduct regular patrols. "The team is expected to travel more than 2,400 nautical miles (4,500 km) during the patrols," Xinhua said.

CNOOC had on June 23 offered nine oil and gas blocks up for foreign bidders in its annual bid round. The acreage overlaps blocks that Vietnam has already awarded off its northern coast, although the entire area is still considered frontier, with scant drilling and no discoveries to date.

In China, local media have lauded the government for finally being proactive in the South China Sea. "China has always been inactive but it has now seized initiative. If the countries involved misjudged China's peaceful tolerance, China still has many cards it can deal," said Huaxun Caijing in an editorial on Monday.

Vietnam, China, the Philippines, Malaysia, Brunei and Taiwan have all claimed sovereignty over overlapping areas in the South China Sea.

Late Friday Chinese foreign ministry spokesman Hong Lei again stressed the government's position at a regular press briefing. Repeating his remarks from Wednesday and Thursday, Hong said China has indisputable sovereignty over the South China Sea islands and their adjacent waters.

"China and Vietnam share a lot of consensus regarding the proper settlement of maritime disputes between them. We hope Vietnam can observe the consensus, refrain from taking actions that may complicate or magnify the disputes and immediately stop its activities in relevant sea areas that infringe upon China's oil and gas rights and interests," he said.

Academics say China's stance is that it would be willing to put aside the various boundary disputes and work toward joint development with its Southeast Asian neighbors.

"China has always insisted this is the best way to come to a resolution," said Tang Qifang at the China Institute of International Studies -- a think tank of the Chinese foreign ministry -- in Beijing. Tang said the latest move involving CNOOC's bid round had been in response to other countries' provocation.

Vietnam's National Assembly on June 21 approved a maritime law claiming sovereignty and jurisdiction over the Paracel and Spratly islands in the South China Sea. China viewed this as a violation of its sovereignty.

"China usually is not the first to change the status quo. It is always defensive, never offensive," said Tang. "China can't stand still and wait for others to explore and produce in the area. We have waited for over a decade and we can't wait any longer."

"The dispute in the South China Sea is not about sovereignty, it is more about energy security," said Zhu Zhiqun, visiting senior research fellow at the East Asian Institute at National University of Singapore. "The problem is that there is deep mistrust among the countries in the region and nobody wants to sit down for constructive dialogue."

"I don't see any immediate resolution in the foreseeable future. Tensions will remain high and it will be a vicious cycle of one country doing something in response to another's actions," Zhu added.

On June 26, CNOOC reported on its website that Chairman Wang Yilin had said at an internal meeting that the company would make renewed efforts to explore for and develop resources in the South China Sea. China has awarded deepwater blocks in undisputed areas to a handful of foreign companies and it hopes that future exploration will yield significant discoveries.

CNOOC plans to kickstart deepwater exploration this year. It started drilling its first independently operated deepwater well in the South China Sea last month with its newly commissioned Haiyang Shiyou 981 semi-submersible rig. Two other deepwater wells will be drilled by the rig later this year.

The company has not responded to email requests for comment.