The US Navy destroyer that sailed near Chinese-claimed islands in the South China Sea last week was under orders from the Third Fleet headquarters in San Diego, a first aimed at bolstering US maritime power in the region, two sources said.

The USS Decatur on Friday challenged China’s “excessive maritime claims” near the Paracel Islands, part of a string of islets, reefs and shoals over which Beijing has territorial disputes with its neighbours.

It was the first time such a freedom of navigation operation has been conducted without the Japan-based Seventh Fleet in command and was a test of changes aimed to allow the US Navy to conduct maritime operations on two fronts in Asia at the same time, two sources told Reuters. The sources spoke on condition that they were not identified.

Having the Third Fleet regularly command vessels in Asia, which it has not done since World War Two, means the US Navy can better conduct simultaneous operations such as on the Korean peninsula and in the Philippines, said one of the sources, who is familiar with the goals of the reorganisation.

“It is the first iteration of what will be a more regular operations tempo,” he said.

The guided-missile destroyer Decatur is part of a three-ship Surface Action Group (SAG) deployed to the South China Sea six months ago, said Commander Ryan Perry, a spokesperson for the Third Fleet in San Diego, who confirmed its command role.

US Pacific Fleet Commander Admiral Scott Swift last year signalled a wider role for the Third Fleet, when he said he was abolishing an administrative boundary along the international dateline that had separated the Third and Seventh fleets.

Until then, Third Fleet vessels crossing the line came under Seventh Fleet command.

This year, an official told Reuters more ships from the Third Fleet would be sent to East Asia.

Bigger frontline role

The reorganisation, giving the Third Fleet a bigger frontline role, comes as momentum for the United States’ Asian “pivot” falters and as Beijing's growing assertiveness fuels tension in the South China Sea.

China claims most of the sea, through which US$5 trillion in ship-borne trade passes a year. The Philippines, Vietnam, Taiwan and Brunei have overlapping claims.

Beijing has accused Washington of deliberately creating tension by sailing its ships close to China’s islands.

Asked about the use of the Third Fleet headquarters, Chinese Foreign Ministry spokesperson Lu Kang said that if US moves harmed the peace, security and stability of the Asia Pacific, then China would naturally oppose them.

“If US moves jeopardise China’s sovereign rights and security interests then China will, when all is said and done, take necessary steps in response,” Lu told a daily news briefing in Beijing, without elaborating.

The latest US operation, its fourth, came as new Philippine President Rodrigo Duterte travelled to China to seek deeper ties with Asia’s biggest economy. This week, he visits US ally Japan.

The Seventh Fleet, headquartered at the Japanese port of Yokosuka near Tokyo, is the most powerful naval fleet in Asia, with some 80 ships, including the United States’ only forward deployed carrier, the USS Ronald Reagan.

The US Third Fleet consists of more than 100 vessels, including four aircraft carriers.

- Reuters