MANILA: The Philippines on Sunday accused China's coast guard of blocking and water-cannoning a Philippine military supply boat in the South China Sea, condemning the "excessive and offensive actions" against its vessels.

China's coast guard countered that it had implemented necessary controls in accordance with the law to deter Philippine ships, which it accused of trespassing and carrying illegal building materials.

China claims sovereignty over almost the entire South China Sea, an assertion rejected internationally, while Malaysia, Vietnam, Brunei, Taiwan and the Philippines have various claims to certain areas.

Beijing often irks its neighbours with maritime actions they call aggressive and with longer-term activities like building islands on reefs and equipping them with missiles and runways.

A Chinese coast guard vessel on Saturday blocked and water-cannoned the chartered Philippine boat on a routine troop rotation and resupply mission, "in wanton disregard of the safety of the people on board and in violation of international law", the Armed Forces of the Philippines said.

It said in a statement the incident occurred near the Second Thomas Shoal, which Manila calls Ayungin Shoal, a submerged reef where a handful of its troops live on a rusty World War Two-era U.S. ship that was intentionally grounded in 1999.

The Chinese coast guard's "dangerous manoeuvres" prevented a second boat from unloading the supplies and completing the mission, it said.

"We call on the China Coast Guard and the Central Military Commission to act with prudence and be responsible in their actions to prevent miscalculations and accidents that will endanger peoples' lives," the armed forces said.

China Coast Guard spokesman Gan Yu responded that China has "indisputable" sovereignty over the Spratly Islands and their adjacent waters, including the Second Thomas Shoal.

"We urge the Philippine side to immediately stop its infringing activities in this waters," Gan posted on the coast guard's WeChat social media account.

The Philippine Coast Guard said the Chinese actions violated laws including two international conventions and a ruling from a global tribunal.

The Permanent Court of Arbitration in The Hague concluded in 2016 that Beijing's expansive claim to the South China Sea was groundless. China maintains it does not accept any claim or action based on the ruling.

The Philippine Coast Guard "calls on the China Coast Guard to restrain its forces, respect the sovereign rights of the Philippines in its exclusive economic zone and continental shelf, refrain from hampering freedom of navigation, and take appropriate actions against the individuals involved in this unlawful incident", said a spokesman, Commodore Jay Tarriela.

After the incident, the U.S. State Department said China's "repeated threats to the status quo in the South China Sea (were) directly threatening regional peace and stability" and that Washington stands with its Philippine allies in the face of such "dangerous actions".

"The United States reaffirms an armed attack on Philippine public vessels, aircraft, and armed forces - including those of its Coast Guard in the South China Sea - would invoke U.S. mutual defense commitments under Article IV of the 1951 U.S. Philippines Mutual Defense Treaty," it said in a statement.