J-20 stealth fighter jet flies at the China's International Aerospace Exhibition in Zhuhai 

China’s stealthy J-20 fighter aircraft are “ready for combat at any time,” according to Yang Wei, a researcher at the Chinese Academy of Sciences.

The aircraft was first commissioned with the People's Liberation Army last fall, making it perhaps the first non-Russian and non-US aircraft to fly with stealth capabilities.

The aircraft, which bears a remarkable resemblance to the F-22 Raptor, is "on par with the fifth-generation jets from both the US and Russia," Yang told the People's Daily. The researcher, who is also the lead J-20 designer, expressed hopes that the jet would faithfully execute any missions with which it is tasked.

One of the biggest struggles for the J-20 has been its engine, which uses a fourth-generation WS-10G engine modeled after Russia's AL-31, the engines that power all Sukhoi Su-27s and Su-27 derivatives. Chinese developers have been hard at work on a WS-15 engine, which has greater capability than the WS-10, and is comparable to the Pratt & Whitney F119 engine that powers F-22 Raptors.

The F-35 and F-22 have the ability to fly at supersonic speeds without turning on their afterburners, while the WS-10 engine in use in the J-10, J-11 and J-20 aircraft must flip on afterburners to hit comparable speeds. Afterburners make planes easier to detect and track, military aviation experts say.

A F-22 Raptor fighter jet of the 95th Fighter Squadron from Tyndall, Florida approaches a KC-135 Stratotanker from the 100th Air Refueling Wing at the Royal Air Force Base in Mildenhall in Britain as they fly over the Baltic Sea towards the newly established NATO airbase of Aemari, Estonia September 4, 2015.

The South China Morning Post reported in February that the J-20 was a long way from being combat ready and that its engine flaws were "embarrassing," citing military sources.

For instance, the WS-15 engines have been in development for a while with modest progress at best. The SCMP source cited a 2015 incident in which the WS-15 engine exploded during a flight test. "It's so embarrassing to change engines for such an important aircraft project several times… just because of the unreliability of the current WS-15 engines. It is the long-standing core problem among home-grown aircraft," of the Post's sources said.