The Philippines said Wednesday it had spotted more concrete blocks allegedly installed by China within Filipino territory in the South China Sea, raising concerns Beijing is planning to build in the disputed waters.

Aerial surveillance has discovered about 75 blocks scattered on a section of the Scarborough Shoal, said defence department spokesman Peter Galvez.

"These can be used for platforms (or) foundations, that is why we said earlier this could be a prelude to any other form of construction," he told reporters.

Chinese foreign ministry spokesman Hong Lei rejected the Philippine allegations.

"What has been said by the Philippines isn't true. Huangyan Island is China's inherent territory," Hong said in an answer to a question posed by state television CCTV at a press briefing, using the Chinese name for the shoal.

"In accordance with the constitution, Chinese government ships maintain routine patrols in waters of Huangyan Island to safeguard our sovereignty over Huangyan Island and to maintain order of relevant waters. That is China's legitimate right and interest and it is beyond dispute."

China claims most of the South China Sea, including waters close to the coasts of its neighbours.

Scarborough Shoal is a small group of reefs and outcrops about 220 kilometres (135 miles) off the main Philippine island of Luzon, within the country's internationally recognised exclusive economic zone. The outcrop is about 650 kilometres from Hainan island, the nearest major Chinese land mass.

The concrete blocks have raised concerns in Manila that China could be planning construction in the waters, as it did in Philippine-claimed Mischief Reef in another area of the sea, in 1995.

"Of course we don't want a repeat of that," Galvez told reporters.

He added that any building work would violate a 2002 non-binding agreement between China and its Southeast Asian neighbours to refrain from actions or hostile acts that could inflame tensions in the flashpoint region.

The Philippines had earlier released an aerial photograph taken Saturday of what it said were about 30 concrete blocks at Scarborough.

A second surveillance flight on Monday photographed more blocks scattered over a two-hectare (4.9-acre) section of the shoal, said Galvez, who did not release the newer photograph.

It was unclear whether the extra blocks were newly laid or were missed by the earlier sweep, he said.

Malaysia, Brunei and Taiwan also have competing claims to parts of the South China Sea, and the rivalries have been a source of tension for decades.

The Philippines engaged China in a tense standoff at Scarborough shoal in 2012.

Manila has said the Chinese had effectively taken control of the shoal by stationing vessels there and preventing Filipino fishermen from entering the area.

In January the government asked a United Nations tribunal to rule on the validity of the Chinese claims to most of the sea.

China has rejected the move, but has said it wanted to solve the dispute through bilateral negotiations with concerned parties.

Philippine foreign department spokesman Raul Hernandez said Wednesday that officials were trying independently to confirm the surveillance photos before lodging an official diplomatic protest.

"Jointly with the (defence department), we are committed to look at ways to appropriately address the issue," he told AFP.

strs-jvg/cgm/pj