A source from the People’s Liberation Army who wished to remain anonymous, said that the suggested ADIZ involves the Pratas, Paracel and Spratly island chains in the contested waterway. The arrangements for the area were as old as those for the East China Sea ADIZ.

Taiwan’s defence ministry said on May 4 that it was aware of the mainland’s strategies.

Lu Li-Shih, a former instructor at Taiwan’s Naval Academy in Kaohsiung, said that the construction and development of artificial islands – particularly the airstrips and radar systems built on Fiery Cross, Subi and Mischief reefs – that had been ongoing for the past several years was all part of Beijing’s ADIZ plan.

“Recent satellite images show that the People’s Liberation Army has deployed KJ-500 airborne early-warning and control aircraft and KQ-200 anti-submarine patrol planes at Fiery Cross Reef,” he said.

He was referring to images obtained by Israel’s ImageSat International and the Asian Maritime Transparency Initiative at the Centre for Strategic and International Studies (CSIS), a Washington-based think tank.

Air-conditioned areas were being built on the reef, suggesting that fighter airplanes – which cannot be exposed to high temperatures, humidity and salinity in the region – would soon be deployed there too, Mr Lu said.

“Once the PLA’s fighter jets arrive they can join the early-warning and anti-submarine aircraft in conducting ADIZ patrol operations.”

Li Jie, a Beijing-based naval expert and retired PLA senior colonel, said that nations normally waited to set out the implementation of an ADIZ until they had the required detection devices, combat capacities and other infrastructure in place to manage it.

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“But the most practical problem is that the PLA has in the past not had the capability to scramble its fighter jets to expel intrusive foreign aircraft in the South China Sea, which is several times the size of the East China Sea, and the cost to support the ADIZ would be huge.”

Chinese authorities in 2010 told a Japanese commission in Beijing that they were thinking of implementing the East China Sea ADIZ.

According to a 2017 document by the CSIS, Beijing said the a discussion was required as its their intentions overlay with Japan’s air defence area.

The report infuriated Tokyo, which retaliated by creating an ADIZ of its own.

It included the Senkaku Islands – known as Diaoyu in Mandarin – a cluster of uninhabited islands in the East China Sea that are divided between Japan, mainland China and Taiwan.

Tensions between Tokyo and Beijing incremented after Japan purchased the Senkakus from a private owner in September 2012.

In response to the move, Beijing laid out its ADIZ in November of the following year.

“China announced the first ADIZ earlier than planned because of the need to assert its sovereignty over the Diaoyu Islands,” Mr Li said.

The announcement face criticism as Japan and the United States condemned the move.