Early versions of the J-20 stealth jet used Russian engines -- but those have since been replaced with domestically-produced twin engines. The jets were first shown to the public with the new Chinese engines last year at Airshow China.
The deployment is intended to "better safeguard China's airspace security and maritime interests," state-run tabloid Global Times reported on Wednesday, citing military experts.
Ren Yukun, a spokesperson for the J-20's state-run manufacturer, added that it was a "training routine" for the J-20 to begin conducting patrols now that it's equipped with Chinese engines, according to Global Times.
The announcement comes just weeks after United States Gen. Kenneth Wilsbach, the commander of US Pacific Air Forces, said US F-35s and Chinese J-20s came into close proximity with each other over the East China Sea.
The East China Sea and South China Sea have both long been contested regions, with overlapping territorial claims by numerous countries.
China claims almost all of the vast South China Sea as its sovereign territory. It has been building up and militarizing its facilities there, turning islands into military bases and airstrips, and allegedly creating a maritime militia that could number in the hundreds of boats.
Meanwhile in the East China Sea, China claims sovereignty over the Japanese-controlled Senkaku Islands, also known as the Diaoyu Islands. In recent years, the US has reiterated its promise to defend the Japanese islands in the event of foreign aggression.
Experts say the deployment of the J-20s shows two things: China's increased confidence in its military abilities, and its warning to other countries with a stake in the territorial dispute.
With some 200 J-20s reportedly in service, the People's Liberation Army Air Force (PLAAF) "now has in regular service a fleet of advanced stealth fighters as good as the Americans, who remain the benchmark," said Peter Layton, a visiting fellow at the Griffith Asia Institute in Australia. He added that China's message to the world is: "Any foreign military aircraft intruding into China's claimed airspace in the East and South China Sea may now be intercepted by J-20s."
While such a move would be politically fraught, the J-20's wide radius of action means it could patrol further out to sea, or stay longer in areas like the East China Sea, Layton said.
Small formations, such as a handful of jets, could also conduct occasional deep patrols into the South China Sea, land to refuel at one of China's island airbases, then return to the mainland.