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 sea, which includes vital shipping lanes through which more than a third of global trade moves.

The new tensions illustrate in stark terms the competition in the South China Sea for what are believed to be sizeable deposits of oil and gas.

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.

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, according to Zhu Feng, a professor of international relations at Peking University, and other Chinese experts.

A website run by PetroVietnam reported the company's exploration vessel Binh Minh 02 had its seismic cable severed by a Chinese fishing vessel last Friday. In May last year, 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.