"I mean, you don't build facilities like that and then not use them," Ian Storey, a senior fellow at the Institute of Southeast Asian Studies Yusof Ishak Institute, told CNN.

A Chinese Ministry of Foreign Affairs spokeswoman said Tuesday she wasn't aware of the report's details but reiterated the Spratly Islands were Chinese territory.

"Whether we decide to deploy or not deploy relevant military equipment, it is within our scope of sovereignty. It's our right to self-defense and self-preservation as recognized by international law," Hua Chunying said.

New hangers, radar almost complete

Fiery Cross, Mischief and Subi reefs are the largest of seven artificial islands built by China in the Spratlys.

China claims the majority of the South China Sea as its territory, despite overlapping claims by a number of other Southeast Asian countries, including the Philippines and Vietnam.

Four bigger hangars have already been completed on Subi Reef, AMTI said, as well as another four on Fiery Cross Reef. Hangars to accommodate five larger planes, such as bombers, were in the final stages of construction on Mischief Reef.

"China's three military bases in the Spratlys and another on Woody Island in the Paracels will allow Chinese military aircraft to operate over nearly the entire South China Sea," AMTI said in a statement.

In addition to the hangars, new radar domes are in various stages of construction on each artificial island, about three arrays on each reef. Fiery Cross, Subi and Mischief reefs now all also have shelters for mobile missiles launchers, according to AMTI.

Air Defense Zone planned?

A similar zone in the South China Sea could rapidly increase tensions in the region, experts said.

"The worry has to be that if China bases its military aircraft (in the South China Sea), they could fly up and challenge anyone's military aircraft or civilian aircraft if they wanted to," said Carl Thayer, regional security analyst and emeritus professor at the University of New South Wales.

China had very rarely enforced its previous ADIZ, and any new zone in the south sea would start out as mostly "symbolic," Storey said.

"And the US will ignore it as it did with the East China Sea ADIZ," he said. "The interesting question is really how the Southeast Asian states will respond."

Planes yet to arrive

Though the infrastructure is almost completed, no military aircraft has been deployed to the islands yet, Thayer and Storey said.

China's next step would be to very slowly deploy planes to the artificial islands to gauge the local and US response, Thayer said.

"What China's going to do is habituate," he said. "You land one there, and then you fly it out, report it in the state media and see what the reaction is."

"Then you add two or three or four, land one and repair it, see what the response is," he said.

South China Sea tensions generally had waned in the past nine months, since Philippines President Rodrigo Duterte took power and sought a closer relationship with China, Storey said.

If China deploys aircraft, "there will be pro forma protests from certain countries, Vietnam in particular. There will be grumbling from certain ASEAN members," he said, referring to the Association of Southeast Asian Nations. "Then, over a period of time, this will become the norm."