Peter Layton, a defense and aviation expert at the Griffith Asia Institute in Australia, noted it was difficult in general to apply labels such as fifth-generation - which includes current stealth jets such as the F-22 and F-35 - to Chinese aircraft, which often include unique design features that never appear again.

China's J-20 and J-35 also have stealthy characteristics but their capabilities are not publicly known and only the J-20 is in service.

The United States has been pouring resources into advanced missiles and other systems to deter China in the Indo-Pacific region. It recently tested a missile interception system on the strategically crucial island of Guam using an advanced radar.

China's daytime flights of the new designs over cities where they could be easily seen was "curious," said Kelly Grieco, a senior fellow at the U.S.-based Stimson Center.