China and the United States exchanged accusations at the weekend over the disputed South China Sea, after China’s military said it had driven away a U.S. warship that the U.S. Navy said was on a routine freedom of navigation operation.

According to a post on the official WeChat social media account of the Chinese People’s Liberation Army Southern Theatre Command on Saturday, the Chinese military deployed its naval and air forces to “track, monitor and warn away” the U.S. destroyer.

The U.S. Navy said on Sunday that the Hopper had “asserted navigational rights in the South China Sea near the Paracel Islands, consistent with international law.”

China claims almost the entire South China Sea, a conduit for more than $3 trillion of annual ship-borne commerce, including parts claimed by the Philippines, Vietnam, Indonesia, Malaysia and Brunei. The Permanent Court of Arbitration in 2016 said China’s claims had no legal basis.

The Philippines and Australia began their first joint sea and air patrols in the sea on Saturday, days after Beijing accused Manila of enlisting foreign forces to patrol the South China Sea, referring to joint patrols by the Philippine and U.S. militaries.

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This weekend’s incident, China said, “proves that the United States is an out-and-out ‘security risk creator’ in the South China Sea.”

Lieutenant Kristina Weidemann, deputy spokesperson for the U.S. 7th Fleet, said in an emailed statement: “The United States challenges excessive maritime claims around the world regardless of the identity of the claimant.

“Unlawful and sweeping maritime claims in the South China Sea pose a serious threat to the freedom of the seas.”

Earlier this month, the United States and China held talks on maritime issues, including the contested South China Sea, where the U.S. underscored concerns about what it called “dangerous and unlawful” Chinese actions, the U.S. State Department said.