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.

China's activity in the contested South China Sea is "hindering" Filipino companies from exploring natural resources in the region, Philippines Foreign Secretary Enrique Manalo said during a talk at Chatham House in London on Tuesday.

China’s military said it drove away three Philippines aircraft “illegally” flying over the disputed Spratly Islands, marking the second aerial incident this week involving Beijing and Manila in the South China Sea.