The latest episode follows an announcement by Hainan province in southern China last week that Chinese vessels would board and search ships in contested areas of the waterway, which includes vital shipping lanes through which more than a third of global trade moves. The new tensions among China, Vietnam and India illustrate in stark terms the competition in the South China Sea for what are believed to be sizeable deposits of oil and gas. Some energy experts in China regard the sea as an important new energy frontier close to home, which could make China less dependent on its huge oil imports from the Middle East. On Monday, China's National Energy Administration named the South China Sea as the main offshore site for natural gas production. Within two years, China aims to produce 150 billion cubic meters of natural gas fields in the sea, a significant increase from the 20 billion cubic meters produced so far, the agency said.

Earlier this year, China's third-largest energy company, state-owned China National Offshore Oil Corporation, launched new equipment that would allow China to drill in deep water for the first time. Since then, CNOOC has been drilling with a rig in deep water in non-disputed waters off the southern coast of China. The escalation in the South China Sea comes less than a month after Xi Jinping took office as China's leader. Mr Xi appears to have taken a particular interest in the South China Sea and the serious dispute between China and Japan over the islands known as Diaoyu in China and as Senkaku in Japan. Whether any of China's most recent actions in the South China Sea were associated with Mr Xi was not clear. But Mr Xi does lead a small group of policymakers clustered in the Maritime Rights Office, which serves to coordinate agencies within China, according to Zhu Feng, a professor of international relations at Peking University, and other Chinese experts. The unit is part of the office of the Foreign Affairs Leading Small Group, Professor Zhu said. The leading small group, now headed by Mr Xi, is widely believed to be China's central policy-making group. China's Foreign Ministry reiterated on Tuesday that China opposed oil and gas development by other countries in disputed waters of the sea. China maintains that it has "undisputed" sovereignty over the South China Sea, and that only China is allowed to develop the energy resources. "We hope that concerned countries respect China's position and rights," the Foreign Ministry spokesman, Hong Lei, said.

Vietnam, which has long been wary of China but enjoys a relationship through its governing Communist Party, summoned the Chinese ambassador on Monday to protest the cutting of the seismic cable, the Vietnamese press reported. A website run by Petro Vietnam, the oil company, reported that the company's exploration vessel Binh Minh 02 had its seismic cable severed by a Chinese fishing vessel last Friday. In May 2011, the Vietnamese authorities said a similar cable of the Binh Minh 02 was cut by three Chinese surveillance ships, resulting in weeks of anti-China protests in Hanoi. In its decree on the new patrols, Vietnam said that civilian ships, supported by marine police and a border force, would be deployed starting next month to stop foreign vessels that violate fishing laws in waters claimed by Vietnam. A senior official in Petro Vietnam, Pham Viet Dung, was quoted in the Vietnamese press as saying that large numbers of Chinese fishing boats, many of them substantial vessels, had recently entered waters claimed by Vietnam. The fishing vessels interfered with the operations of the oil company, he said.

India, whose state-run oil company, Oil and Natural Gas Corporation, has a 45 per cent interest in exploration with Petro Vietnam, also reacted strongly. The head of the Indian navy, Admiral D.K. Joshi, said India was prepared to send navy vessels to protect its interests in the sea. "Now, are we preparing for it? Are we having exercises of that nature? The short answer is 'yes'," Admiral Joshi told reporters in India. The most recent moves by China in the South China Sea have not won total support at home. Professor Zhu said he did not believe that China had become more assertive in the South China Sea. But, he said: "The cable cutting is really unfriendly." New York Times, AP