Beijing will never give up its claims to South China Sea islands, its foreign minister insisted Wednesday ahead of a state visit to the US by President Xi Jinping, after a Washington think-tank said it may be building its third airstrip in the area.

China claims almost the whole of the sea and over the past year has asserted its stance by rapidly converting tiny reefs into artificial islands, with facilities for military use.

Taiwan, Vietnam, the Philippines, Malaysia and Brunei all have rival claims to the waters, which incorporate strategically crucial shipping lanes and could harbour oil and gas deposits.

The Pentagon has warned that Beijing's activities are changing the regional status quo, and has weighed sending warships and surveillance aircraft within 12 nautical miles -- the normal territorial zone around natural land -- of the new artificial islands.

Foreign minister Wang Yi stressed that China has no intention of backing down on its claims.

"I wish to reiterate here that (the) Nansha Islands are China's territory," he said in a speech to foreign diplomats in Beijing, using the Chinese name for the Spratly islands.

"This is fully backed by historical and legal facts," he added. "It is totally understandable for China to uphold its own territorial sovereignty and prevent its legitimate interests from infringement."

Work began last year on a 3,000-metre (9,842 feet) runway on Fiery Cross reef in the Spratlys, around 1,000 kilometres from China's island province of Hainan.

It was now "well advanced" said the Washington-based Center for Strategic and International Studies (CSIS) on Tuesday.

Satellite photos of another reef, Subi, where nearly four million square metres (988 acres) of land have been reclaimed, show grading work and possible runway construction is being carried out, it said.

And satellite photos taken last week show that a retaining wall has been built on Mischief Reef, creating a 3,000-metre rectangular area, with a cement plant set up, CSIS said, "suggesting another runway could be in the works".

The images appear to contradict a claim by China in August that its reclamation activities had stopped.

Mischief Reef is only 21 nautical miles from Second Thomas Shoal, where the Philippines -- whose defence budget is a fraction of China's -- deliberately grounded a landing ship in 1999 to serve as a makeshift base for a contingent of marines.

"A third airstrip on Mischief Reef... would complete the triangle, significantly boosting China's air patrol and interdiction capabilities over the contested waters and features of the Spratlys," wrote Gregory Poling of CSIS' Asia Maritime Transparency Initiative (AMTI).

It would heighten tensions and present "greater operational headaches for all the claimants as well as outside players like the United States", he added, as Chinese President Xi Jinping prepares to make his first state visit to the US next week.

- 'Already stopped' -

Airstrip building in the Spratlys goes back nearly 40 years and four other claimants already have such facilities, although China's are much longer and could be used by any of the People's Liberation Army's aircraft, analysts say.

Beijing says its reclamations and facilities are intended for civilian and well as military purposes, with Wang saying the intent was to fulfil China's "international obligation".

China was "committed to maintaining peace and stability in the South China Sea and assuring freedom of navigation and overflight", he said.

The latest satellite pictures -- showing continuing dredging and channel widening -- were taken after Wang said at an ASEAN summit in Kuala Lumpur last month that land reclamation works were over.

Bonnie Glaser of AMTI said the works "underscore Beijing's unwillingness to exercise self-restraint and look for diplomatic paths to reduce tensions with its neighbours, the United States, and other nations".

"Beijing appears to be sending a message to President Barack Obama that China is determined to advance its interests in the South China Sea even if doing so results in heightened tensions with the United States," she added.