China has conducted the first test-flight of its J-20 stealth fighter, Chinese President Hu Jintao has confirmed to US Defence Secretary Robert Gates. The confirmation came after images of the 15-minute flight in Chengdu appeared on several Chinese websites. Mr Hu said that the flight had not been timed to coincide with Mr Gates' visit, the US defence secretary said. But both Russia and China are known to be working on prototypes of stealth fighters, which are invisible to radar. Mr Gates' three-day visit to Beijing comes amid US concern over the speed at which China's military is modernising and upgrading its technology.