SYDNEY (Reuters) - Australia and the United States began their biggest ever joint military exercises on Thursday, a show of force, largely at sea, aimed at sending a message both to allies and potential foes, including China.

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The exercises involving 33,000 U.S. and Australian troops on board battleships equipped with strike jets, comes as tension over China’s more assertive activity, particularly in the disputed South China Sea, has raised fears of confrontation.

Admiral Harry Harris, head of the U.S. Pacific Command, asked how he thought China would view the exercises, said the size of the deployment was intended as a signal.

“I’m pleased about that message it sends our friends, allies, partners and potential adversaries,” Harris told reporters on board the USS Bonhomme Richard.

Relations between the United States and China have soured in recent months as the United States seeks to counter what it perceives as Chinese assertiveness in the Pacific, encapsulated by Beijing’s artificial island building in the South China Sea.

China claims most of the resource-rich South China Sea, through which about $5 trillion in ship-borne trade passes every year. Brunei, Malaysia, the Philippines, Vietnam and Taiwan also have claims on the waterway.

The United States estimates China has added more than 3,200 acres (1,300 hectares) of land on seven reefs and tiny isles in the South China Sea over the past three years, installing runways, ports, aircraft hangars and communications equipment.

James Curran, professor of politics and foreign policy at the University of Sydney, said the exercise illustrated the close military ties between the United States and Australia but could raise worry in China about being surrounded.

“When there are concerns about China’s activities in the South China Sea, this military exercise will send a signal and take on greater significance,” Curran said.

“China will be concerned if it looks like containment, when you have Australia, the United States and others trying to gang-up on China.”

The exercise will go on for a month in Australian territorial waters and will include training in land and air operations.