The Philippines says the China Coast Guard has fired water cannon and "sideswiped" a government vessel in the South China Sea.
The incident happened near the disputed Scarborough Shoal and Beijing said it acted "in accordance with the law".
The US ambassador to the Philippines condemned China's "unlawful use of water cannons and dangerous manoeuvres".
The South China Sea is at the centre of a territorial dispute between China, the Philippines and other countries. But tensions between Manila and Beijing have sharply escalated in the last year.