The Philippines released photographs Thursday to back its claim that China was reclaiming land on a disputed reef in the South China Sea in an apparent effort to build an airstrip.

Manila warned on Wednesday that China may be building an airstrip on the Chinese-held Johnson South Reef -- claimed by the Philippines and Vietnam -- boosting the superpower's claim to most of the strategic Asian waters.

A series of photographs released by the foreign department Thursday appeared to show large-scale reclamation in stages. The latest photograph dated March 11, 2014 appears to show a large light-coloured landfill, surrounded by shallow turquoise waters.

"This series of photographs... from Philippine intelligence sources, shows in stages the extensive reclamation by China on Mabini Reef (Johnson South Reef)," the foreign department said in a statement.

"These actions are considered destabilising and in violation of the Declaration on the Conduct of Parties in the South China Sea (DOC) and international law," it said, asserting that the reef was "part of Philippine territory".

Chinese foreign ministry spokeswoman Hua Chunying on Wednesday would not confirm Manila's claim, but asserted the outcrop was Chinese territory.

The Philippines said it 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.

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

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

Manila says the reef falls within the country's 370-kilometre (200-nautical-mile) exclusive economic zone under a United Nations convention.

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.

The Philippines in March asked a United Nations tribunal to declare what Manila said was China's claim to 70 percent of the sea as illegal.

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 the United States and triggered the worst anti-China protests in Vietnam for decades.