The Philippines will "vigorously defend what is ours", President Ferdinand Marcos said on Saturday, as Manila's maritime row with Beijing over the South China Sea escalates.

Marcos told a Philippine Military Academy graduation ceremony that the Southeast Asian country faced "a blatant disregard of internationally accepted principles".

His remarks follow a series of incidents involving Philippine and Chinese vessels near disputed reefs in the South China Sea that have strained diplomatic ties.

Marcos ordered the graduates to "keep our people safe in their homes, our territory secure, our defences against threats strong, and our democracy stable".

"Against intruders who have been disrespecting our territorial integrity, we will vigorously defend what is ours," Marcos said in the speech, which did not specifically mention China.

China Coast Guard vessels have used water cannon against Philippine boats several times in recent months in the contested waters, where there have also been collisions.

Beijing claims most of the South China Sea, brushing off rival claims from other countries, including the Philippines, and an international ruling that its assertion over the waterway has no legal basis.

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

A Philippine civilian convoy sailed towards Scarborough Shoal to bring fuel and food supplies for Filipino fishermen in the area this week. It later turned back after one of the boats was shadowed by a Chinese navy ship.

China defends its actions by saying that it takes necessary measures against vessels it says are infringing on its territory.

Marcos said the Philippines' conduct would be "always guided by law and our responsibility as a rules-abiding member of the community of nations".

He has said that Manila will not respond in kind to the use of water cannon against its vessels.