WASHINGTON • China appears to be carrying out preparatory work for a third airstrip in contested territory in the South China Sea, according to a US expert who cited satellite photographs taken last week.

The photos taken on Sept 8 for a Washington think-tank, the Centre for Strategic and International Studies (CSIS), show construction on Mischief Reef, one of seven artificial islands China has created in the Spratly archipelago.

The images show a retaining wall around an area 3,000m long, matching similar work by China on two other reefs in the Spratlys, Subi and Fiery Cross, Mr Greg Poling, director of CSIS' Asia Maritime Transparency Initiative, said on Monday.

He said the work "more likely than not indicates preparations for a runway" on the reef. Satellite photographs from late June showed that China had almost finished a 3,000m airstrip on Fiery Cross.

Mr Poling said other satellite photos from last week showed that work was advancing at Subi Reef, where "clearly, what we have seen is going to be a 3,000m airstrip and we have seen some more work on what is clearly going to be some port facilities for ships".

Asked about Mischief Reef on Monday, Foreign Ministry spokesman Hong Lei repeated China's claim to"indisputable sovereignty" over the Spratly islands and its right to establish military facilities there.

Security experts say that 3,000m airstrips would be long enough to accommodate most Chinese military aircraft, giving Beijing greater reach into the heart of maritime South-east Asia.

News of the work comes ahead of a visit to Washington next week by Chinese President Xi Jinping. US worries about China's increasingly assertive territorial claims are expected to be high on the agenda.

US Defence Department spokesman Bill Urban declined to comment on Mr Poling's assessment, but repeated US calls for a halt to land reclamation, construction and militarisation of South China Sea outposts to "ease tensions and create space for diplomatic solutions".

A new airstrip at Mischief Reef would be particularly worrying for the Philippines.

It would allow China to mount "more or less constant" patrols over Reed Bank, where the Philippines has long explored for oil and gas, Mr Poling said.

- REUTERS