The Philippines on Thursday accused Chinese Coast Guard vessels of firing water cannon at boats delivering supplies to Filipino marines in the disputed South China Sea, and ordered Beijing to "back off".

Foreign Secretary Teodoro Locsin said he had expressed "outrage, condemnation and protest" to Beijing over the incident, which he said happened Tuesday as the Philippine boats were travelling to Second Thomas Shoal in the contested Spratly Islands.

"Fortunately, no one was hurt; but our boats had to abort their resupply mission," Locsin said in a statement on Twitter, describing the three Chinese vessels' actions as "illegal".

Locsin described the Philippine boats as "public", suggesting they were civilian vessels, and said they were covered by a mutual defence pact with the United States.

"China has no law enforcement rights in and around these areas," he added. "They must take heed and back off."

Tensions over the resource-rich seas spiked this year after hundreds of Chinese vessels were detected at Whitsun Reef, which is also in the Spratly archipelago.

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.

-'We do not ask permission' -
China controls several reefs in the South China Sea including Scarborough Shoal -- which Beijing seized from Manila in 2012 -- and is just 240 kilometres (150 miles) west of the main Philippine island of Luzon.

It has asserted its stance by building up small shoals and reefs into military bases with airstrips and port facilities.

After China occupied Mischief Reef in the mid-1990s, the Philippines marooned a derelict navy vessel atop the nearby Second Thomas Shoal to assert Manila's territorial claim. Members of the Philippine Marines are based there.

Locsin said the shoal was within the Philippines' Exclusive Economic Zone, and warned China's "failure to exercise self-restraint threatens the special relationship" between the two countries. 

"We do not ask permission to do what we need to do in our territory," he said.

The Chinese embassy in Manila did not immediately respond to a request for comment.

Outgoing President Rodrigo Duterte has sought to pivot away from the United States, the Philippines' former colonial master, towards China since taking power in 2016 and has appeared reluctant to confront Beijing.

But facing growing domestic pressure to take a harder line, Duterte has insisted Philippine sovereignty over the waters is not negotiable.

Cabinet Secretary Karlo Nograles said Thursday: "We will continue to assert our sovereignty... over our territory."

In July, Duterte walked back on a decision to axe a key military deal -- the Visiting Forces Agreement -- with the United States during a visit by Pentagon chief Lloyd Austin.

In a joint statement issued this week, the two countries reaffirmed "our treaty commitments" that include "obligations to respond to an armed attack in the Pacific Area on either the United States or the Philippines."