From listening posts to jet fighter deployments and now surface-to-air missiles, China's expanding facilities in the Paracel Islands are a signal of long-term plans to strengthen its military reach across the disputed South China Sea.
Diplomats and security experts in contact with Chinese military strategists say Beijing's moves to arm and expand its long-established holdings in the Paracels will likely be replicated on its man-made islands in the more contentious Spratly archipelago, some 500 kms (300 miles) further south.
Eventually, both disputed island groups are expected be used for jet fighter operations and constant surveillance, including anti-submarine patrols, while also housing significant civilian populations in a bid to buttress China's sovereign claims.
Crucially, that would give Beijing the reach to try to enforce any effective air defence zone in the South China Sea, similar to the zone it created over the East China Sea in late 2013.
US officials confirmed on Thursday the "very recent" placement of surface-to-air missiles on Woody Island, the site of the largest Chinese presence on the Paracels, criticising the move as contrary to China's commitments not to militarize its claims in the South China Sea.
Beijing says it is entitled to "limited defensive facilities" on its territory, and dismissed reports about the missile placement as media "hype".
Ian Storey, a South China Sea expert at Singapore's ISEAS Yusof Ishak Institute, said he believed similar weapons could be deployed to China's holdings in the Spratlys within a year or two.
"This would enable China to back up its warnings with real capabilities," he said.
Bonnie Glaser, a military analyst at the Centre for Security and International Studies in Washington, said the Paracels build-up was a likely precursor to similar military deployments on China's recent reclamations in the Spratlys.
While Chinese officials might use on-going US operations in the South China Sea as justification, "there is a plan that has been in place for quite some time", Glaser said.
The HQ-9 missile batteries, guided by radar tracking systems, have a range of 200 km (125 miles) and are the most significant defensive weapon China has yet placed on the Paracels, regional military attaches say.
The move could complicate surveillance patrols carried out routinely by US and Japanese aircraft as well as flights by US B-52 long-range bombers, operations China objected to last November.
It could also challenge operations by Vietnam's expanding fleet of Russian-built SU-30 jet fighters.
China's expansion of the Paracels, which it has occupied since forcing the navy of the-then South Vietnam off the islands in 1974, pre-dates its moves to begin large-scale reclamations on seven reefs in the Spratlys three years ago.
It landed fully-armed jet fighters on an expanded airstrip on Woody Island in November, and reinforced hangars have been completed, regional diplomats said.
Coast guard and fishing facilities have also been expanded, along with fuel storage tanks and housing for more than 1,000 civilians in what was declared "Sansha City" in 2012, Chinese analysts say.
Radar coverage and other electronic surveillance equipment has also been improved, and analysts expect the Paracels to play a key part in protecting China's nuclear armed submarine fleet on Hainan Island, 200 km to the north.
Speaking privately, Vietnamese officials say it is now far more difficult for their fishing fleets and coast guard to get close to the Paracels as they try to assert their own sovereign claims.
A similar build-up in the Spratlys would give China its first permanent military presence deep in the maritime heart of Southeast Asia, military attaches say.
China claims most of the South China Sea and while Vietnam and Taiwan also claim both archipelagoes in their entirety, the Philippines, Malaysia and Brunei also claim part of the Spratlys.
The waterway carries some $5 trillion in seaborne trade each year.
HIGHER STAKES IN SPRATLYS
Chinese officials have repeatedly stressed the civilian nature of the Spratlys expansion, including lighthouses, search and rescue bases and environment research stations.
Three runways have recently been completed and China last month announced the first successful test landings of civilian airliners on the new 3,000-metre airstrip at Fiery Cross reef.
Chinese analysts say the first military flights from the Spratlys could start within months.
Wu Shicun, the head of China's National Institute for South China Sea Studies, said lessons learnt from the Paracels expansion could be transferred to the Spratlys, particularly to manage water supplies and waste.
"There is no real dispute in the Paracels...so the development on the Paracels has been much faster and governance has also been more complete," he said.
Yanmei Xie, a Beijing-based security analyst with the International Crisis Group think-tank, said China would seek to exploit dual-use facilities, such as radars and runways, on the Spratlys but would be cautious about openly deploying military assets.
"The Spratly Islands are more complicated because they involve every claimant," she said. "It can be more costly to China diplomatically and geopolitically."