Philippine forces retook an island in the South China Sea Wednesday in the first such combat exercise in the disputed waters as Chinese navy ships kept watch from a distance, the Philippine military chief said.

Gen Romeo Brawner Jr, who witnessed the drills from a navy frigate, said the exercise underscored the readiness of Filipino forces to defend the country’s sovereignty at all costs.

“We are warning our neighbours or whoever or any external forces that we are capable of defending our islands,” Brawner told a small group of journalists, including The Associated Press, invited to the exercises.

There were no immediate comments from Chinese officials, but they have opposed past war drills in the South China Sea, which Beijing claims virtually in its entirety.

The long-seething territorial disputes that also involve other claimants are a delicate fault line in the US-China rivalry in Asia and likely will remain a major foreign policy concern for the next American president.

China has rapidly expanded its military and become increasingly assertive in pursuing territorial claims in the South China Sea. The tensions have led to more frequent confrontations, primarily with the Philippines and Vietnam, though the Chinese coast guard has had recent territorial tiffs with Indonesia and Malaysia.

In the drills, a Philippine navy frigate approached Loaita Island, which the Philippines calls Kota Island, while four speedboats with marines and navy sailors landed on its beach. A Philippine air force aircraft later dropped food and other supplies to the forces that retook the island.

Brawner said Chinese navy ships observing from a distance “added realism to the exercise.”

The US has been regarded as a crucial counterweight to China and has supported the Philippines as it faces Chinese aggression.