Philippine forces recaptured an island in the South China Sea on Wednesday in the first combat exercise of its kind in the disputed waters, as Chinese navy ships watched from a distance, the Philippine military chief said.

Gen. Romeo Brawner Jr, who watched the drills from a navy frigate, said the exercise underscored the Philippine forces’ resolve to defend the country’s sovereignty at all costs.

“We are warning our neighbors or anyone or any outside force that we are capable of defending our islands,” Brawner told a small group of journalists, including The Associated Press, invited to the exercises.

There was no immediate comment from Chinese officials, but they have opposed previous war exercises in the South China Sea, which Beijing claims virtually in its entirety.

Long-running territorial disputes that also involve other contenders constitute a delicate fault line in the U.S.-China rivalry in Asia and will likely remain a major foreign policy concern for the next U.S. president.

China rapidly expanded its military and became increasingly assertive in pursuing its territorial claims in the South China Sea. Tensions have led to more frequent clashes, mainly with the Philippines and Vietnam, although China’s coast guard has recently had territorial disputes with Indonesia and Malaysia.

During the exercises, a Philippine Navy frigate approached Loaita Island, which the Philippines calls Kota Island, while four speedboats carrying marines and sailors landed on its beach. A Philippine Air Force plane then dropped food and other supplies to forces that retook the island.

Brawner said Chinese naval ships observing remotely “added realism to the exercise.”

The United States is seen as a crucial counterweight to China and has supported the Philippines in the face of Chinese aggression.