MANILA, Philippines (AP) — Chinese coast guard ships blocked and sprayed a powerful stream of water at two Philippine boats carrying supplies to troops at a disputed South China Sea shoal, prompting Manila to order Beijing’s ships to back off and warn that its supply vessels are covered by a mutual defence treaty with the United States, Manila’s top diplomat said yesterday.

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 Second Thomas Shoal, which lies off western Palawan province in the Philippines’ internationally recognised exclusive economic zone.

Locsin said in a tweet that the three Chinese coast guard ships’ actions were illegal and urged them “to take heed and back off”.

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 assertions, ratcheting up tensions and alarming rival claimants and Western governments led by the US.

The Philippine Government 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, worked to nurture.

In Beijing, Chinese Foreign Ministry spokesperson Zhao Lijian said China’s coast guard had upheld China’s sovereignty after the Philippine ships entered Chinese waters at night without permission.

National Security Adviser Hermogenes Esperon told reporters in Manila that two Chinese coast guard ships blocked the two wooden-hulled supply vessels. A third coast guard ship “water cannoned the two for one hour,” he said, adding that the outrigger of one of the Philippine ships was displaced by the impact.

A water cannon is a device on firetrucks that shoots high-velocity water for firefighting and crowd control but has also been installed on Chinese coast guard ships to drive away what China considers intruders in the waters it claims.

The Philippines plans to deploy coast guard and Bureau of Fisheries vessels instead of navy ships to back up its forces and enforce its fishing laws at Second Thomas Shoal, which Filipinos call Ayungin and China refers to as the Ren’ai reef, Esperon said.

The number of Chinese surveillance ships has increased in recent weeks in the far-flung shoal and also around Thitu, a larger Philippine-occupied island in the Spratly archipelago in the South China Sea’s most hotly contested area, he said.

Philippine forces won’t be deterred from travelling again to the shoal following the incident, he said.

“We will continue the resupply and we do not have to ask the permission of anybody because that is within our territory,” Esperon said.

The Philippine military deliberately ran aground a World War II-era warship, the BRP Sierra Madre, at the submerged shoal in 1999 to fortify its claim and provide a shelter to a small contingent of Filipino marines.

The Sierra Madre is now effectively a rusty shipwreck but the Philippine military has not decommissioned it. That makes the ship an extension of the government and means any attack on the ship is tantamount to an assault against the Philippines.