U.S. Defense Secretary Robert Gates has said the Chinese may be further ahead in developing a stealth jet dubbed J-20 that military experts believe could change the balance of power in Northeast Asia.

"We knew they were working on a stealth aircraft," Gates told reporters aboard an aircraft bound for China on Saturday. "I think that what we've seen is that they may be somewhat further ahead in the development of that aircraft than our intelligence had earlier predicted."

Video and stills of the J-20 have been circulating on the Internet since Dec. 22, sparking controversy among Chinese and foreign experts over their authenticity.

China's new stealth jet, dubbed the Jian-20 /Hong Kong's Phoenix Satellite TV

The Wall Street Journal on Monday carried an article headlined "China Stealth Jet 'Leak' Viewed as Intentional." It said, "Despite China's tight Internet controls, Chinese bloggers have posted dozens of images of the J-20 online in the week leading up to a long-delayed visit by Robert Gates... It is no accident that the pictures were published just before Mr. Gates arrives in China."

Gates said China apparently wanted to show that its technology has developed much more than the U.S. had thought. The images revived controversy over Gates' decision to cut funding for the U.S.’ own ultramodern stealth jet, the F-22, in 2009, the paper added.

Based on analysis of the images, experts speculate that the J-20 is undergoing its final test in preparation for a test flight.

Japanese media quoted a Chinese military source as predicting the jet will be ready for its first test flight in mid-January. The Chinese government and military have made no comment, and the state media merely relayed some overseas reports or quoted Chinese experts' "personal opinions."