The Philippines said Tuesday it had lodged a formal protest after two Filipino vessels were prevented from bringing supplies to marines by the Chinese coastguard on a disputed shoal in the South China Sea.

The foreign ministry said the Chinese charge d'affaires was summoned over the March 9 incident on Second Thomas Shoal, which sits around 200 kilometres (125 miles) from the western Philippine island of Palawan and is part of the contested Spratly island group.

"China's actions constitute a clear and urgent threat to the rights and interests of the Philippines," the Department of Foreign Affairs said in a statement.

It said the two Philippine-flagged civilian ships had been contracted by the Philippine navy to deliver supplies and equipment as well as replacement troops to the isolated outcrop.

China's foreign ministry spokesman Qin Gang said the two vessels "infringed China's territorial sovereignty" and "violated" the code of conduct for the South China Sea.

Philippines Foreign Department spokesman Raul Hernandez later told reporters that two Chinese coastguard boats blocked the Philippine vessels and used "signboard, sirens and megaphones" to order them to leave the area.

A tiny unit of marines living on a decrepit, beached Philippine naval vessel and only connected to the outside world by satellite phones has been guarding the shoal since the late 1990s amid an increasingly tense territorial dispute.

Hernandez said the Philippines had been supplying its forces to the shoal for 15 years without any interference from China.

Referred to by China as Ren'ai Reef and by the Philippines as the Ayungin Shoal, the Second Thomas Shoal is claimed by the Philippines, China and Taiwan.

Malaysia, Brunei and Vietnam also claim other parts of the Spratlys, which lie near vital sea lanes, rich fishing grounds and are also believed to sit on vast mineral resources.

- Complex dispute -

Gang told reporters: "According to the Chinese coast guard, yesterday (its) fleets on regular patrol found two Philippine-flagged vessels carrying construction materials approaching the Ren'ai Reef.

He added "The Chinese coastguard's vessels spoke to the two vessels, and the two vessels left the waters around the Ren'ai Reef that afternoon."

He insisted that the Philippines had repeatedly been asked to remove the grounded vessel on the reef, but had failed to do so.

The incident comes amid heightened tensions between the two neighbours over their conflicting claims to territory in the South China Sea.

On February 25, the Philippines formally protested China's alleged use of water cannon to drive away Filipino fishermen who were approaching Scarborough Shoal, another South China Sea outcrop about 22 kilometres from the main Philippine island of Luzon.

China is embroiled in several territorial disputes with its neighbours including the Philippines and Japan, with tensions centred on rival claims in the South China Sea and East China Sea.

The Philippine government has sought United Nations arbitration to settle the dispute, but China has rejected the move.

strs-mm/cgm/jom/lto