China plans to boost patrols in the South China Sea, converting retired naval ships and possibly even drafting in fishing boats to protect its interests in the disputed area.

The announcement comes weeks after the United States said that one of its unarmed maritime surveillance ships had been harassed by five Chinese naval boats in waters about 75 miles (120km) off the southern Chinese island of Hainan. China said that the US ship was engaged in spying. The Pentagon then sent in a destroyer to protect the USNS Impeccable as it carried out its surveys in the region.

Wu Zhuang, director of the Administration of Fishery and Fishing Harbour Supervision of the South China Sea, said: “China will make the best use of its naval ships and may also build more fishery patrol ships, depending on the need.”

He did not specify if the boats would be armed when they are sent out into a region of atolls, islands and reefs that are some or all disputed by China, Taiwan, the Philippines, Vietnam, Malaysia and Brunei. The boats will be sailing in some of the world’s busiest shipping lanes. More than half the globe’s oil tanker traffic passes through the South China Sea since it offers the shortest route between the Pacific and Indian oceans for ships bringing energy from the Middle East to China and Japan.

Mr Wu said that the situation in the region was becoming increasingly complicated. "Faced with a growing amount of illegal fishing and other countries' unfounded territorial claims of islands in China’s exclusive economic zone, it has become necessary to step up the fishery administration's patrols to protect China’s rights and interests.”

His reference to China’s rights comes as the People’s Liberation Army is implementing an ambitious naval modernisation plan. This week it sent its largest and most advanced fishery patrol ship, the Yuzheng 311, to waters around the Spratly Islands, a move seen as a response to the United States and also to renewed Philippine claims.

The last time the Chinese Navy engaged in battle was in 1996, when three of its ships had a brief shootout with a Philippine gunboat in the South China Sea. Two years later, the Philippine Navy arrested Chinese fishermen off Scarborough Shoal.

But the timing, just after the incident involving a US ship, has been seen as an attempt by China to show who is in charge in these disputed waters.

Craig Snyder, of Australia’s Deakin University, said: "This is about the Chinese wanting to stop the US 'spying' in waters that are close to China. not so much an attempt by China to extend its naval reach but to deny access to the US Navy of Chinese coastal waters. Yes, the Chinese ultimately want to exercise sea control or at least sea denial throughout the South China Sea, but at this point they simply want to make the Americans think twice when operating in and around Chinese waters."

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