The Philippine military said Tuesday that Chinese boats illegally "seized" food and medicine airdropped to a Filipino outpost in the South China Sea.

The alleged incident happened on May 19 at Second Thomas Shoal in the Spratly Islands, where Filipino troops are garrisoned on a grounded navy vessel to assert Manila's claims to the waters.

Beijing claims almost the entire South China Sea and there has been a series of confrontations involving Chinese and Philippine vessels near contested reefs, often during Philippine resupply missions to Second Thomas Shoal.

The Philippine military accused China of "aggressive and unprovoked interference" when two Chinese rigid-hulled inflatable boats allegedly came within 10 metres of the Sierra Madre vessel and seized an airdropped package meant for Filipino troops.

It was the first time supplies had been seized, the military said.

"This action of getting or confiscating our supplies is illegal," military chief General Romeo Brawner told reporters.

"You're not supposed to confiscate the supplies of another country, even in war."

Chinese personnel on board the boats later dumped the items in the water, Philippine Navy spokesman for the West Philippine Sea Commodore Roy Vincent Trinidad said.

It was not clear if they belonged to the Chinese coast guard or navy, the military said.

The Filipino troops were able to retrieve most of the items that were airdropped that day, the military said.

- 'Stop making trouble' -

China hit back Tuesday, insisting the Sierra Madre was illegally grounded on the reef and urging the Philippines to "stop making trouble".

"The Philippine side has also repeatedly provoked and intensified conflicts, escalating the situation. This is unacceptable to us," Chinese foreign ministry spokeswoman Mao Ning told reporters.

Second Thomas Shoal is about 200 kilometres (120 miles) from the western Philippine island of Palawan, and more than 1,000 kilometres from China's nearest major landmass, Hainan island.

Philippine resupply missions were usually by sea, but Brawner said last month they did an airdrop to avoid "resistance" and "harassment".

Brawner denied reports that Filipino troops on board the Sierra Madre had pointed their weapons at the Chinese boats.

China brushes off rival claims to the South China Sea from other countries, including the Philippines, and ignores an international ruling that its claims have no legal basis.

To assert its stance, Beijing deploys coast guard and other boats to patrol the waters and has turned several reefs into artificial islands that it has militarised.