China said Friday it had "supervised" a Philippine ship as it delivered supplies to a grounded vessel at a disputed reef in the South China Sea, after a string of tense encounters in the area this year.

Beijing claims almost all of the economically vital waterway and continues to press its assertions there despite an international tribunal ruling that they have no legal basis.

Tensions between China and the Philippines have flared in recent months during a series of confrontations in the waters around the contested Second Thomas Shoal and Sabina Shoal.

However, in July, the two sides said they had reached a provisional deal on resupply missions to a Philippine ship, the Sierra Madre, which is grounded on the Second Thomas Shoal with a garrison on board, aimed at asserting Manila's claims to the reef.

Beijing's coast guard said on Friday that Manila had conducted a resupply mission "in accordance with the provisional agreement".

The civilian vessel "delivered daily necessities to the illegally 'grounded' warship on Ren'ai Reef", spokesperson Liu Dejun said, using the Chinese name for the shoal.

Liu said the coast guard "questioned and confirmed the (identity of the) Philippine vessel and supervised the whole process".

"It is hoped that the Philippines will keep its promise, meet China halfway, and jointly control the maritime situation," he said.

A spokeswoman for the Philippine armed forces confirmed that Manila "was able to deliver essential supplies to our troops in the area".

"We confirm the presence of Chinese vessels in the vicinity of Ayungin Shoal during our execution of (the) mission," Colonel Francel Padilla said, using the Philippines' term for the reef.

"Up to eight different Chinese vessels were monitored in the vicinity, but posed no threat," she said.

"We remain vigilant and prepared to respond to any challenges in protecting our personnel and operations."

In June, a Filipino sailor lost a thumb in a confrontation near the Second Thomas Shoal when Chinese coast guard members wielding knives, sticks and an axe foiled a Philippine Navy attempt to resupply its troops.

Chinese and Philippine vessels have collided at least three times recently near Sabina Shoal, located 140 kilometres (86 miles) from the Philippines' western island of Palawan and 1,200 kilometres from China's nearest major landmass, Hainan island.

Another Philippine vessel, the Teresa Magbanua, was anchored on that shoal in April to assert Manila's claims but left the area this month.

The Chinese coast guard did not mention Sabina Shoal in its Friday statement but said it would continue to enforce "rights protection" around the Spratly Islands, of which the reef is a part.

After the Teresa Magbanua's departure, Beijing said its sovereignty over the shoal was "indisputable".