A China Coast Guard ship (top) and a Philippine supply boat engaging in a stand off at South China Sea. File/AFP

Chinese coast guard ships blocked and sprayed a powerful stream of water on two Philippine ships carrying supplies to troops at a disputed South China Sea shoal, prompting a warning to Beijing that the vessels are covered by a mutual defense treaty with the United States, Manila’s top diplomat said on Thursday.

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

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The Philippines on Thursday condemned "in strongest terms" the actions of three Chinese coast guard vessels that it said blocked and used water cannon on resupply boats headed towards a Philippine-occupied atoll in the South China Sea.

"China has no law enforcement rights in and around these areas. They must take heed and back off," Locsin said in a statement, reminding China that a public vessel is covered by a Philippines-United States Mutual Defense Treaty.

Locsin said he had conveyed "in the strongest terms" to China's ambassador in Manila "our outrage, condemnation and protest of the incident."

A Chinese Coast Guard vessel is seen patrolling in the South China Sea. File/AP

China's embassy did not immediately respond to Reuters' request for comment.

China claims almost all of the sea, through which trillions of dollars in trade passes annually, with competing claims from Brunei, Malaysia, the Philippines, Taiwan and Vietnam.

The contested waters also have valuable fishing grounds and are believed to sit atop vast oil and gas deposits.

Beijing has ignored a 2016 ruling by The Hague-based Permanent Court of Arbitration that its historical claim over most of the sea to be without basis.

It was 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 South China Sea and has transformed seven shoals into missile-protected island bases to cement its claims, ratcheting up tensions and alarming rival claimants and Western governments led by the US.

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.