Professor Carlyle A. Thayer, photo from the Australian Defence Force Academy.

 "This is a demonstration that at short notice China can deploy similar systems in the Spratlys on the pretext of a threat from the United States, Thayer said in an interview with DTiNews.

According to Thayer, up until now concerns by the United States and other countries about militarisation of the South China Sea have focused on China's artificial islands in the South China Sea. Woody Island in the Paracels has been militarised for quite some time. Its airfield was recently extended to 3,048m. Woody houses hangars and military aircraft. This past year the latest model J-11 BH/BHS jet fighters were deployed there. An off-shore island houses a major SIGINT base. Other photos that have appeared recently shows a marked step up in construction on two sites - North Island and Tree island - in the Paracels’ Amphitrite island group.

"The deployment of a very sophisticated air defence system is no doubt in response to US aerial activities and the recent freedom of navigation operational patrol near Triton Island," he said. "Triton is part of the Paracels but is considered an outlier because it is located to the west of the main group of islands in the Paracels."

Woody Island is shown in satellite images taken on February 14, 2016 and February 3, 2016, in this handout image provided by ImageSat International N.V. 2016, on February 18, 2016.

China's actions raise the stakes and risks for future US maritime reconnaissance patrols in waters surrounding the Paracels. The HQ-9 system could also threaten carrier based planes coming to the assistance of a US Navy warship that was confronted by China during a future freedom of navigation exercise in the Paracels.

How has China illegally militarised the Paracel Islands?

According to the Australian expert, China constructed 1,200 foot runway on Woody island that has been extended twice (not including the extension in 2015). The airstrip on Woody Island can accommodate fighter aircraft such as the Su-27 and Su-30MKKs, H-6 bombers and large supply transport aircraft. The facilities adjacent to the runway include four hangers. Air traffic is controlled by a Type 791 X-band precision-approach radar. Other military infrastructure on Woody Island includes naval docks capable of accommodating frigates and destroyers and a fuel depot. PLA soldiers are based on Woody Island to protect the runway and other military facilities.

China has also built military-related facilities elsewhere in the Paracels. A weather station has been built on Pattle Island, while Robert Island houses a radio beacon, the only beacon south of Hainan. The docks on Duncan Island are being expanded. A Signals Intelligence (SIGINT) station has been operating on Rocky Island, the highest premonitory, since 1995.  This station could provide air or surface warning and support air missions or ship targeting. Open sources report that China may have stationed the HY-2 anti-ship cruise missile on Woody Island.