China has deployed more ships to a disputed shoal in the South China Sea amid a tense stand-off with the Philippines, officials and state media said on Wednesday.

As of Monday night, there were five Chinese government vessels -- up from three -- and 16 fishing boats in the area, the Philippine foreign department said.

Manila has lodged a fresh protest with the Chinese embassy over the build-up, department spokesman Raul Hernandez said.

Using the shoal's Philippine name, Hernandez added: "The Philippines, therefore, demands that China's vessels immediately pull out from Bajo de Masinloc and the Philippines' exclusive economic zone."

China's official Xinhua news agency said controls have been "strengthened" in the area and quoted a foreign ministry spokesman as saying it had "about 20 fishing boats" near the disputed Scarborough Shoal, "roughly the same number as in previous years".

China claims the shoal along with most of the South China Sea, even up to the coasts of its Asian neighbours, while the Philippines says the shoal is well within its 200-nautical-mile exclusive economic zone.

Cranking up tensions, both countries have had ships posted around the shoal since early April, when Chinese vessels prevented a Philippine Navy ship from arresting Chinese fishermen.

The two claimants had vowed to de-escalate the tensions and both imposed separate fishing bans in the area from May 16, while Philippine President Aquino stopped a planned protest trip to the shoal by a Philippine ex-soldier.

Hernandez said Chinese fishermen appeared to be breaking their own ban.

"They are fishing and collecting corals," he said.

He said the two governments were still in talks over the dispute, and the alleged Chinese build-up only served to "escalate tension" around the shoal.

Taiwan, Brunei, Vietnam and Malaysia also claim parts of the South China Sea.

The rival claims have for decades made the waters one of Asia's potential military flashpoints.