BEIJING—The first clear pictures of what appears to be a Chinese stealth fighter prototype have been published online, highlighting China's military buildup just days before U.S. Defense Secretary Robert Gates heads to Beijing to try to repair defense ties.

The photographs, published on several unofficial Chinese and foreign defense-related websites, appear to show a J-20 prototype making a high-speed taxi test—usually one of the last steps before an aircraft makes its first flight—according to experts on aviation and China's military.

The exact origin of the photographs is unclear, although they appear to have been taken by Chinese enthusiasts from the grounds of or around the Chengdu Aircraft Design Institute in western China, where the J-20 is in development. A few experts have suggested that the pictured aircraft is a mock-up, rather than a functioning prototype of a stealth fighter—so-called because it is designed to evade detection by radar and infrared sensors.

But many more experts say they believe the pictures and the aircraft are authentic, giving the strongest indication yet that Beijing is making faster-than-expected progress in developing a rival to the U.S. F-22—the world's only fully operational stealth fighter.

China's defense ministry and air force couldn't be reached to comment on the latest photos. Even without official confirmation, however, the photographs are likely to bolster concerns among U.S. officials and politicians about China's military modernization, which also includes the imminent deployment of its first aircraft carrier and "carrier-killer" antiship ballistic missiles.