The US Navy on Monday called on Beijing to clarify its intentions in the South China Sea, expressing concern that China's use of coast guard vessels to assert its claims in the disputed waters could lead to unwanted hostilities.

The US and Chinese navies have an agreed code to avoid flare-ups during unexpected encounters in the busy waterway, but the protocols does not include the coast guard and non-naval vessels, Vice Admiral Joseph Aucoin, commander of the US 7th Fleet, said during a press briefing in Singapore.

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"I would really like to see China to be more transparent on what their intentions are," said Aucoin. "I think that will relieve some of the angst we are seeing."

Aucoin likewise warned that any move by China to fly jet fighters from runways on its man-made islands in the South China Sea would be destabilizing, and would not deter US flights over the area.

Civilian Vessels

China's white-hulled coast guard vessels have become the vanguard for Beijing's claims over the South China Sea, relegating the bigger, more heavily armed grey-hulled warships of the People's Liberation Army Navy (PLAN) to less visible roles in Beijing's efforts to assert authority over the territory.

The distinction is important.  By deploying its coast guard to the contentious territory, experts say Beijing is reinforcing the political message that virtually all of the South China Sea is China's sovereign territory, subject to its domestic laws.

The US Department of Defense estimates that around 30 percent of the world's ship-borne trade transits across the South China Sea, including around $1.2 trillion in trade bound for the US.

Analysts claim that China's use of its coast guard in disputed areas of the South China Sea is also designed to ensure Beijing wins international sympathy in case of an encounter with the US Navy. 

"They would spin it as, 'Here is the United States, the source of all our problems in the South China Sea, bullying our civilian coast guard vessels in an area where we have legitimate jurisdictional rights,'" Ian Storey, a senior fellow at the Institute of Southeast Asian Studies in Singapore, told Bloomberg recently.

Operational Airfield

Vice Admiral Aucoin said the US and Chinese navies routinely hold high-level dialogues, and that the two institutions have a good working relationship.  

He admitted, however, that he worries about the involvement of the Chinese coast guard and other civilian vessels in Beijing's claims over the South China Sea, saying he is "not sure about their professionalism."

The US Office of Naval Intelligence reports that China has amassed the largest coast guard fleet in Asia, with an estimated 205 maritime law enforcement vessels.  China has built a new giant coast guard ship -- nicknamed "The Beast" -- which it is likely to arm with machine guns before deployment to the South China Sea.  

"How do we approach that when it is not grey hull versus another grey hull, when it's other types of ships?" said Aucoin. "I think we'll see more of that in the future."

Chinese coast guard vessels usually operate around the reefs on which China has constructed runways, lighthouses and other buildings.

Security analysts have said that Beijing will probably begin using the runways for military operations in the coming months.

"They do have an operational airfield but I don't know when they will start flying fighter-type aircraft out of there," said Aucoin. "We will fly, sail and operate wherever international law permits, and that includes flying over that airspace."

TagsUS-China relations, Territorial disputes in the South China Sea, Coast Guard ships