A Philippine plane warned a Chinese military helicopter over the South China Sea that it was "flying too close," as tension between the U.S. ally and Beijing soars.

A Chinese military helicopter flew within 10 feet (3 meters) of a Philippine patrol plane over the South China Sea on Tuesday, in what observers said was the second incident of potentially catastrophic behavior by the People’s Liberation Army against foreign aircraft in a week.

Disputed South China Sea airspace that Beijing claims as its own falls within Manila’s Exclusive Economic Zone, where the Philippines and other countries enjoy freedom of overflight.