MANILA, PHILIPPINES - Chinese coast guard ships blocked and used water cannons on two Philippine supply boats heading to a disputed shoal occupied by Filipino marines in the South China Sea, provoking an angry protest to China and a warning from the Philippine government that its vessels are covered under a mutual defense treaty with the United States, Manila's top diplomat said Thursday.

Philippine Foreign Secretary Teodoro Locsin Jr. said no one was hurt in the incident in the disputed waters on Tuesday, but the two supply ships had to abort their mission to provide food supplies to Filipino forces occupying the Second Thomas Shoal, which lies off western Palawan province in the Philippines' internationally recognized exclusive economic zone.

Locsin said in a tweet that the three Chinese coast guard ships' actions were illegal and he asked them 'to take heed and back off.'

The Philippine government has conveyed to China 'our outrage, condemnation and protest of the incident,' Locsin said, adding that 'this failure to exercise self-restraint threatens the special relationship between the Philippines and China' that President Rodrigo Duterte and his Chinese counterpart, Xi Jinping, have worked hard to nurture.

There was no immediate comment from Chinese officials in Manila or Beijing.

Long-running dispute

The incident is the latest flare-up in the long-simmering territorial disputes in the strategic waterway, where China, the Philippines, Vietnam, Malaysia, Brunei and Taiwan have overlapping claims. China claims virtually the entire waterway and has transformed seven disputed shoals into missile-protected island bases to cement its claims, ratcheting up tensions and alarming rival claimants and Western governments led by the U.S.

Washington has no claims in the busy waterway but has patrolled the region with its Navy ships and aircraft to assure its allies, including the Philippines, and ensure freedom of navigation and overflight. China has repeatedly warned the U.S. to stay away from the disputed waters and not meddle in what it says is a regional issue.

President Joe Biden and his predecessor, Donald Trump, have repeatedly assured the Philippines that the U.S. will honor its obligation under the two nations' Mutual Defense Treaty if Philippine forces, ships or aircraft come under attack in the long-disputed region.