China's creeping militarisation of the South China Sea has advanced another step, with surveillance balloons creating an early warning system for its illegal island fortresses.
It's another example of Beijing's slow-but-steady lockdown of the contested waterway.
Each new move is just below the trigger point of an international incident but further entrenches its dominance over its neighbours, news.com.au reported.
Its latest move is apparent in a high-resolution photograph released by Israeli satellite company ImageSat International. It shows a balloon floating over Mischief Reef in the Spratly Islands.
It has an enormous airstrip. It has dozens of bomb-hardened hangars. It has sufficient underground fuel tanks and ammunition dumps to withstand a prolonged siege.
But since its completion in 2017, China's Mischief Reef artificial-island fortress has not played host to fighters, bombers or surveillance aircraft.
Instead, its arrays of anti-aircraft and antimissile weapons stand guard over what China insists is a rescue and fishing facility.
It's a diplomatic tightrope. But Beijing recognises that the high-profile presence of combat aircraft would put the lie to its assertion that it is not militarising the South China Sea.
Instead, it's doing everything it can to ensure any overnight deployment of its warplanes is met by the most combat-ready facilities possible. And that means flying into totally-observed airspace.
It also brings China one step closer to totally locking down the 425,000sq km Spratly Archipelago.
EYES IN THE SKY
The presence of an airborne warning and control system aircraft on any of China's island fortresses would spark international outrage. They're big. They have large crews. They also usually fly with fighter escorts.
A balloon, however, is innocuous, even though its capabilities may exceed those of its winged cousin.
These balloons carry heavy payloads. They can stay aloft as long as weather permits.
And that payload can be anything from phased-array radars to infra-red and optical sensors or any mix of electronic jamming and surveillance devices.
Mischief Reef is the closest to the Philippines of seven artificial island fortresses built in the Spratly Islands by China. In essence, these balloons complete an overlapping network of radars and satellites reaching far into the South China Sea to detect low-flying aircraft and small vessels.
The balloons add to Beijing's ability to lockdown the region – if desired.
Mischief Reef has been the subject of 21 Freedom of Navigation operations by the United States Navy. These aim to enforce the International Court of Arbitration's 2016 ruling that the artificial islands did not represent sovereign territory.
The USS Gabrielle Giffords and USS Wayne E. Meyer recently passed within 12 nautical miles (22km) of Mischief Reef. That's the UN-recognised territorial boundary China claims over the seized tidal outcrop.
"These missions are based on the rule of law and demonstrate our commitment to upholding the rights, freedoms and lawful uses of the sea and airspace guaranteed to all nations," said a spokeswoman for the US Navy's Seventh Fleet.
Beijing said it "tracked, monitored, identified, warned and expelled" the frigate from its territory.
The People's Liberation Army (PLA) said it "sent ships and aircraft to conduct the whole-process monitoring and verification on the two US warships and warned them to leave".
A PLA spokesman accused the US of attempting "to stir up trouble". "We absolutely oppose any country using freedom of navigation and flight as an excuse to undermine China's sovereignty and security," the spokesman said.
Even as the two US warships moved through the area, China's first home-built aircraft carrier (known only as Type 001A) passed close to Taiwan as it headed into the South China Sea.
"US actions have seriously undermined China's sovereignty and security interests and damaged peace and stability in the South China Sea. China strongly opposes such actions and has lodged stern representations with the US," China's Foreign Ministry spokesman Geng Shuang warned after the recent incident.
US Secretary of Defence, Mark Esper, responded by accusing China of "increasingly resorting to coercion and intimidation to advance its strategic objectives". He added it was time for the region to push back.
It's a message Beijing is taking seriously.
The PLA has voiced growing confidence in its ability to close down the South China Sea air and sea space: "China will take all measures necessary to adamantly safeguard national sovereignty and peace and maintain peace and stability in the South China Sea," its spokesman said. "No matter what trickery the US pulls from its hat, the Chinese military has the resolution and capability to protect its national sovereignty and security as well as maintain peace and stability in the region."