State-run China Central Television and one English-language news outlet said last week the military had deployed domestically built J-11B jet fighters to Woody Island in the Paracel archipelago.
Hangars in the island chain would improve China's overall control of the sea, the television network said, quoting a Chinese military expert. The country eventually could intercept foreign aircraft, it said.
Five other governments, all militarily weaker than China, contest Beijing's claims to about 90 percent of the 3.5 million-square-kilometer sea that's prized for fisheries, marine shipping lanes and undersea fossil fuel reserves.
Artilleries are fired during a military drill in Qingtongxia, Ningxia Autonomous Region, China September 25, 2017.
China's aircraft deployment will raise alerts among the other claimants as well as the United States, which has more fire power than China and insists the sea remain open to all, analysts believe.
But China and Southeast Asian states, which are used to Chinese maritime maneuvers and recipients of Chinese economic support, are expected to remain friendly.
Friendly gestures, assertive initiatives
In November China agreed to negotiate with the 10-member Association of Southeast Asian Nations (ASEAN) on a code to prevent accidents at sea by 2018. An official from Beijing said then that China would ensure freedom of navigation for the Southeast Asian states.
China probably intends to stick to both pledges, analysts say.
Beijing is mixing "friendly gestures" with "assertive initiatives," said Fabrizio Bozzato, a Taiwan Strategy Research Association fellow who follows Southeast Asia.
"At the end of the day, their South China Sea policy remains unchanged," Bozzato said. "They regard the South China Sea as Chinese waters. It seems to me they have a clear intention to make the South China Sea or most of it Chinese waters by what we could say 2030."
Leaders from left to right, Malaysia';s Prime Minister Najib Razak, Myanmar';s State Councellor and Foreign Minister Aung San Suu Kyi, Thailand';s Prime Minister Prayut Chan-ocha, Vietnam';s Prime Minister Nguyen Xuan Phuc, U.S. President Donald Trump, Philip
Brunei, Malaysia, the Philippines and Vietnam assert sovereignty over parts of the same sea, overlapping China's claims. Taiwan also claims the whole sea.
Since 2010 China has angered its neighbors by using landfill to build up islets and installing other military hardware to bolster control.
But to reinforce friendships, China has helped the Philippines develop infrastructure since the two sides became friendlier last year, pumped up tourism to Vietnam and invested heavily in Brunei and Malaysia.
Muscle flexing in a time of calm
Upbeat talks with neighbors do not stop China from bulking up its air defense, scholars say.
International organizations have spotted Chinese aircraft in the Paracel chain at other times over at least the past year, making this case unique because official Chinese media reported the deployment, said Jonathan Spangler, director of the South China Sea Think Tank in Taipei.
"China was trying hard to offset all the negative publicity it was getting and also trying to bolster its image with ASEAN countries and trying to work things out with the Philippines under (President) Duterte," Spangler said.
"Perhaps now the attention in the region has been drawn a bit away from the South China Sea temporarily and that could be (a) reason China might be more willing to be a bit more outspoken," he said.
Resistance against U.S. aircraft
China probably will not start any fights with the deployment of fighter jets, said Yun Sun, East Asia Program senior associate at the Stimson Center think tank in Washington.
It seldom uses an air defense identification zone (ADIZ) in the East China Sea to stop foreign aircraft as feared when the zone was created in 2013.
The military will instead check the South China Sea for foreign aircraft and ask make them to leave, Sun said. It would especially target U.S. aircraft, she added. In 2001 a Chinese pilot and a U.S. intelligence aircraft got into a mid-air collision as the Chinese side tried to make the other land, setting off a political flap.
A Chinese J-11 fighter jet is seen flying near a U.S. Navy P-8 Poseidon about 215 km (135 miles) east of China';s Hainan Island in this U.S. Department of Defense handout photo taken Aug. 19, 2014.
"The Chinese are going to harass warplanes, especially American planes in the area, but I don't think their purpose will be to shoot anyone down or stop anyone from coming to the airspace of that region, because they know they cannot stop the Americans." Sun said.
"I think it's a substantive step that strengthens the Chinese ability of maneuver,' she said.
China may eventually step up vigilance against three U.S. allies with a shared interest in checking Beijing's maritime expansion, Sun added. They are Australia, India and Japan. China isn't sure what to make of their intentions yet, she said.
Vietnam might get mad
The jet fighter deployment avoids the South China Sea's Spratly Islands, which are disputed by five other governments, picking instead a chain that's actively contested only by Vietnam. Vietnam had not protested as of Wednesday.
Use of the 130-islet Paracel chain for air defense may indicate China believes that claim is stronger than its hold over the Spratly archipelago, Sun said. Beijing has controlled the Paracel chain since the 1970s. About 1,000 Chinese people live on Woody Island.
China's follow-up to the deployment will depend on what reactions follow, Spangler said. A formal complaint from abroad would hurt China's image as neighborly negotiator, but lack of one would offer "smooth sailing" until someone protests, he said.