A Chinese FC-31 stealth fighter has its test flight ahead of the 10th China International Aviation and Aerospace Exhibition in Zhuhai, South China's Guangdong Province, Nov 10, 2014. File photo: Xinhua

New photos of a prototype FC-31, China's second type of stealth fighter jet, have been appearing frequently on Chinese social media since May, years after disappearing from the public eye. Now painted in a silver gray coating, its development is making smooth progress, experts said on Monday.

A new set of photos of what seems to be an FC-31 fighter jet on a test flight was posted on Sina Weibo on Saturday. This is not the first time the aircraft has made an appearance recently, as some photos were also posted by another Sina Weibo user on May 18, Shanghai-based news outlet eastday.com reported.

Unlike photos taken in previous years, the FC-31 prototype in the new photos is painted with silver gray coating, eastday.com said. It seems to be an upgraded version with modifications made to its aerodynamic design just like the prototype that made its maiden flight in 2016, instead of the original version that made its public debut at Airshow China in 2014.

The authenticity of the photos, including the time and location they were taken, cannot be verified.

The photos soon sparked heated discussions among military enthusiasts, as they were reposted on forums on military affairs and overseas social media like Twitter.

Fu Qianshao, a Chinese air defense expert, told the Global Times that the new painting could be a sign that the FC-31 was testing its stealth capability and low-observability against the naked eye.

Fu said that while the photos could not show exactly how much progress had been made, the aircraft is confirmed to be conducting new test flights and making significant steps.

Based on the results of the test flights, improved prototypes could be made, Fu said.

Military observers have long speculated that the made-for-export FC-31 could be put into domestic military service. Some claimed an upgraded FC-31 could serve as China's next-generation carrier-based fighter jet.

The Chinese Air Force, Navy and foreign clients could all be interested in this advanced stealth fighter jet, Fu said, noting that the FC-31 will likely continue its development and be equipped with new engines and devices in the future.

The FC-31 is a single-seat, twin-engine multi-role fighter jet catering to the demands of future battlefield environments. It is 17.3 meters long and has a wingspan of 11.5 meters, according to an info flyer by its maker Aviation Industry Corporation of China (AVIC) in 2018.