The Philippines warned on Wednesday that China may be building an airstrip on a reef in the South China Sea, boosting the superpower's claim to most of the strategic Asian waters.

Filipino surveillance aircraft have been monitoring large-scale reclamation and earthmoving activity on Chinese-held Johnson South Reef since January, the defence department said.

Asked if China was building an airstrip on the reef, also claimed by the Philippines and Vietnam, Foreign Secretary Albert del Rosario said: "That's one possibility".

Chinese foreign ministry spokeswoman Hua Chunying, would not confirm the Philippine claim, but asserted the outcrop was Chinese territory.

"Whatever construction China carries out on the reef is a matter entirely within the scope of China's sovereignty. I don't know what particular intentions the Philippines has in caring so much about this," she said at a regular press briefing Wednesday.

Last week, the Chinese press downplayed the activity at the reef, saying it was merely to renovate the living facilities for troops stationed there.

"We can confirm that there is ongoing reclamation or earthmoving activities in that portion," Filipino defence department spokesman Peter Galvez told reporters Wednesday.

"It has been getting bigger and bigger."

Del Rosario told reporters the Philippines had filed a diplomatic protest against China's reclamation works on the reef last month, but Beijing rejected it on grounds the reef is part of Chinese territory.

The Philippines calls the outcrop the Mabini Reef, while China calls it Chigua Reef. Internationally, it is recognised as the Johnson South Reef.

It is part of the Spratly chain, and is located about 300 kilometres (186 miles) west of the large western Philippine island of Palawan.

China seized the reef and other outcrops from Vietnam in a deadly 1988 skirmish.

- Strained ties -

It is not the first time the Philippines has made allegations against China over construction at disputed outcrops in the sea.

In September last year, Manila accused Beijing of laying concrete blocks on disputed Scarborough Shoal that it said could be a "prelude to construction".

However, in an embarrassing about-face, Manila dropped the allegations weeks later after concluding that the concrete blocks were previously-existing structures.

The Philippines said China took effective control of the shoal in 2012, stationing patrol vessels and shooing away Filipino fishermen, after a stand-off with the Philippine Navy.

Beijing's claim to nearly all of the South China Sea, which straddles vital sea lanes and is believed to sit on vast oil and gas reserves, has strained its ties with neighbours.

Earlier this month, Vietnam accused China of ramming its ships in an encounter near another part of the sea where Beijing had deployed a deep-sea oil rig.

Those actions were described as "provocative" by US Secretary of State John Kerry in a phone call to Chinese Foreign Minister Wang Yi.

The Philippines in March filed a formal plea to the United Nations challenging Beijing's alleged territorial claims to about 70 percent of the South China Sea, in defiance of Chinese warnings that it would seriously damage their already-frayed relations.

Manila contends that, under international law, it has exclusive rights to exploit the resources of waters and outcrops within its "exclusive economic zone", defined as those within 370 kilometres (200 nautical miles) of its coast.

Beijing has rejected UN arbitration and urged Manila to settle the dispute through bilateral talks instead.

The Chinese claims to the sea also overlap those of Taiwan as well as Brunei and Malaysia.

Meanwhile, the Philippines said Wednesday two of the 11 Chinese fishermen arrested last week by Filipino police in another area of the Spratlys were flown to Guangzhou late Tuesday.

Manila filed charges against their nine colleagues for poaching and collecting protected species, but freed the two because they are minors.