China's Behaviour in South China Sea must be Challenged - U.S. Navy Official

China's "aggressive behaviour" in the South China Sea, including the use of water canon by its coast guard against a Philippine vessel, must be challenged and checked, the commander of the U.S. Navy's Seventh Fleet said on Sunday.

Vice Admiral Karl Thomas assured the Philippines of U.S. backing in the face of "shared challenges" in the region, saying: "My forces are out here for a reason."

The largest of the U.S. Navy's forward-deployed fleets, the Seventh Fleet, headquartered in Japan, operates as many as 70 ships, has around 150 aircraft and more than 27,000 sailors.

It operates over an area of 124 million square km (48 million square miles) from bases in Japan, South Korea and Singapore.

"You have to challenge people I would say operating in a grey zone. When they're taking a little bit more and more and pushing you, you've got to push back, you have to sail and operate," Thomas told Reuters.

"There's really no better example of aggressive behaviour than the activity on 5 August on the shoal," he added.

On Aug. 5, a Chinese coast guard ship used water cannon against a Philippine boat carrying supplies to troops aboard a warship Manila intentionally grounded on a shoal in the South China sea, a fault line in the rivalry between the U.S. and Beijing in the region.

Thomas said he had had discussions with Vice Admiral Alberto Carlos, the head of the Philippine Western Command overseeing the South China Sea, "to understand what his challenges are to find opportunities to be able to help him".

"We certainly shared challenges. So I wanted to better understand how he views the operations that he's responsible for. And I want to make sure that he understood what I had available," said Thomas who was in Manila for a port call.

On Saturday, Thomas said he joined a flight from Manila "to go out and check out the South China Sea."

The Philippines won an international arbitration award against China in 2016, after a tribunal said Beijing's sweeping claim to sovereignty over most of the South China Sea had no legal basis.

China has built militarised, manmade islands in the South China Sea and its claim of historic sovereignty overlaps with the exclusive economic zones of the Philippines, Vietnam, Malaysia, Brunei and Indonesia.

The Chinese Embassy in Manila did not immediately respond to a request for comment.

(Reuters - Reporting by Karen Lema; Editing by Nick Macfie)