Satellite photographs taken last week show that China appears to be building a third airstrip in the South China Sea's contested territory.
The photographs allegedly taken for Washington's Centre for Strategic and International Studies think tank show construction on Mischief Reef, one of several artificial islands that China has created on the Spratly archipelago.
According to Greg Poling, the director of the centre's Asia Maritime Transparency Initiative, the images show a rectangular area with a retaining wall, 3,000 meters long, matching similar work by China on two other reefs - Subi and Fiery Cross.
"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," he added.
Reuters said according to security experts, the air strip would be long enough to accommodate most Chinese aircraft, effectively giving Beijing greater reach into the heart of maritime Southeast Asia.
Commander Bill Urban, a spokesman for the US Defence Department declined to comment specifically on Poling's assessment of the satellite photographs, but repeated earlier US calls for a halt to land reclamation, construction and militarization of South China Sea outposts to "ease tensions and create space for diplomatic solutions.
"China's stated intentions with its programme, and continued construction, will not reduce tensions or lead to a meaningful diplomatic solution," he added.
Poling said that the new airstrip would be particularly worrying for the Philippines which also has claims in the area and has explored for oil and gas.
Reuters said that Hong Lei, China's Foreign Ministry spokesman when asked about the airstrip on Monday, repeated China's claim to "indisputable sovereignty" over the Spratly Islands and its right to set up military facilities there.