THE Peoples Liberation Army is building an airstrip on a third artificial island in the South China Sea on a mainline trade route ahead of China's President Xi Jinping's trip to Washington next week, reports the New York Times.

The newspaper cited photographs released by the Centre for Strategic and International Studies, which show preparation for airfields on submerged reefs in the contested Spratly islands that the centre says are being transformed into islands. 

An airstrip on Mischief Reef is about 20 miles from a small Philippine garrison on an existing tiny island and will put the installation under great pressure, said James Hardy, Asia-Pacific editor of IHS Jane's Defence Weekly.

The airstrip will most likely be used for turboprop patrol, but it could easily be equipped for "full military action" if needed, Mr. Hardy said. 

The most important function of the strip, he said, will be another site for listening devices and early warning radar, much like the technology being installed on Woody Island in the Paracel Islands of the South China Sea, which are also contested. Evidence of that will probably appear soon on Mischief Reef, he said.