Beijing reaffirmed its right to build on disputed islands in the South China Sea on Thursday after satellite imagery emerged of construction operations turning tropical reefs into concrete artificial islands.

The newly released images prompted concern from the United States, which warned China that its island building activity posed a threat to regional stability.

"In our view, China's land reclamation and the construction activity are fuelling greater anxiety within the region," State Department spokesman Jeff Rathke told reporters.

Rathke said Washington is concerned that China "might militarise outposts on disputed land features of the South China Sea.

"So we are watching these developments closely and we continue to raise our concerns with China as well as with others in the region to urge all parties to avoid destabilising activities," Rathke said.

The Philippines -- one of the most vocal of China's neighbours in defending its competing territorial claim -- reacted strongly, calling for the Asian giant to "dismantle" the reclaimed land.

"They have to dismantle it," said Peter Paul Galvez, spokesman for Manila's defence department. "It is a concern not only of our country and region but of the whole international community."

A series of satellite images posted on the website of the Center for Strategic and International Studies show a flotilla of Chinese vessels dredging sand onto Mischief Reef and the resulting land spreading in size.

Before-and-after images of other outcrops in the Spratly Islands record runways appearing from jungle, smooth-sided solid masses where coral once lay, and man-made harbours replacing natural reefs.

Analysts say the pictures show how China is attempting to create facts in the water to bolster its sovereignty claims.

Beijing asserts sovereignty over almost the whole of the South China Sea, including areas close to the coasts of other littoral states, using a nine-segment line based on one that first appeared on Chinese maps in the 1940s.

The Philippines, Vietnam, Malaysia, Brunei and Taiwan all have overlapping claims.

- 'Indisputable sovereignty' -

"China exerts indisputable sovereignty over the Nansha islands and affiliated waters," said foreign ministry spokeswoman Hua Chunying, using the Chinese name for the islands, which literally means "Southern Sand".

"Such construction is totally within China's sovereignty, and it is legitimate, sensible and lawful. It does not influence nor target any specific country."

The works were to "safeguard the territorial sovereignty and maritime rights and interests of China", she said, adding: "We will build more civilian facilities."

The Philippines has taken its sovereignty claim to the United Nations for arbitration, a process rejected by Beijing.

Manila has troops stationed on some islands it controls, which also have civilian residents.

"As we have mentioned more than once, actually since this administration started, we have been warning everyone of the implications of their (China's) actions, of their aggressive means so like today, these reclamations... will have further implications in the long term," defence spokesman Galvez told AFP.

The South China Sea is home to strategically vital shipping lanes and is believed to be rich in oil and gas, and the territorial dispute has raised concerns in Washington, with the US asserting that freedom of navigation is in its national interest.

The new satellite photographs were taken by Digital Globe, a commercial provider of satellite images, and analysed by CSIS.

"It appears that China's building projects are part of an expansive territorial grab or to make China's disputed Nine-Dash Line claim a reality," US Navy Lieutenant Commander Wilson VornDick wrote in an analysis on the CSIS site.

The director of the centre's Asia Maritime Transparency Initiative, Mira Rapp-Hooper, told the New York Times: "China's building activities at Mischief Reef are the latest evidence that Beijing's land reclamation is widespread and systematic."

US admiral Harry Harris last month reportedly said that Chinese reclamation efforts in the area had created more than four square kilometres of artificial landmass.