MANILA — T he Philippines accused Chinese maritime officials on Friday of carrying out an “unjustified assault” on Vietnamese fishermen in the contested waters of the South China Sea, adding its voice to a fraught dispute over the confrontation. Vietnam said this week that Chinese law enforcement officers had beaten 10 fishermen and seized their gear while they were working last Sunday near the Chinese-controlled Paracel Islands, which Hanoi also claims and calls Hoang Sa. China, which claims most of the busy waterway, said on Tuesday the fishermen had been there illegally and that it had taken steps to stop them.

It did not respond immediately to the Philippine statement. Other recent run-ins between China and the Philippines, a U.S.

treaty ally, have made the highly strategic South China Sea a potential flashpoint between Washington and Beijing. Philippine National Security Adviser Eduardo Ano said on Friday his country stood with Vietnam in denouncing Sunday’s “egregious act”. “Such use of force against civilians blatantly violates international law .

.. and breaches basic human decency,” he added in his statement.

The Philippines and Vietnam also have overlapping claims on islands in the South China Sea, but both agreed in August to work more closely together and resolve disagreements peacefully. Their coastguards held their first joint exercises that month. Encounters have become more frequent in the past year as Beijing presses its claims and Manila refus.