WASHINGTON (Reuters) - China is still years away from being able to field a stealth aircraft, despite the disclosure of images indicating that it appears to have a working prototype, Pentagon officials said on Wednesday.

The images have been posted on a number of websites and were published on the front page of The Wall Street Journal on Wednesday. The Pentagon said they appeared to show a Chinese J-20 stealth fighter prototype making a high-speed taxi test.

The disclosure of the photographs comes just days before U.S. Defense Secretary Robert Gates is due to travel to Beijing on Sunday, and analysts could only speculate about the motives for their sudden appearance.

“This might be just a way of demonstrating that whatever obstacles there might have been (to China developing these technologies), they’ve overcome them,” said Randy Schriver, a China expert and former State Department official for Asia.

The pictures are likely to heighten concerns about China’s military buildup, including possible deployment in 2011 of its first aircraft carrier and a new anti-ship ballistic missile seen as a threat to U.S. aircraft carriers.

Some analysts say that the J-20 photos, if authentic, are a strong indicator that China is making faster-than-expected progress in developing a rival to Lockheed Martin’s F-22 Raptor, the world’s only operational stealth fighter designed to evade detection by enemy radar.

But U.S. Vice Admiral David Dorsett, director of naval intelligence, said deployment of the J-20 was years away.

“It’s still not clear to me when it’s going to become operational,” he said. “Developing a stealth capability with a prototype and then integrating that into a combat environment is going to take some time.”

He dismissed any suggestions that the Pentagon had underestimated China’s stealth capability.

Pentagon spokesman Colonel Dave Lapan said China was still having problems with the engines for its previous generation of fighter jets.

“Our assessment of when China might have an operational fifth generation fighter puts it at some point in the future, close to the end of this decade,” Lapan said.

PREMIER U.S. FIGHTER

A U.S. intelligence official estimated in May that the J-20 could rival the F-22 Raptor within eight years.

The Raptor is the premier U.S. fighter, with cutting-edge “fifth-generation” features, including shapes, materials and propulsion systems designed to make it appear as small as a swallow on enemy radar screens.

The United States hopes to field a successor to the F-22, known as the F-35, in the coming years, and news of faster-than-expected Chinese stealth technology could add pressure on Lockheed Martin and the Pentagon to speed development.

The revelation comes on the heels of warnings from the head of U.S. forces in the Pacific about China’s new anti-ship ballistic missile, which could target U.S. aircraft carriers.

U.S. officials acknowledge China has moved faster than expected in developing the missile and is now in a position to start deploying it.

Dorsett said it would be a mistake to underestimate China’s military advances, fueled by its fast-growing economy. At the same time, their military capabilities were only a shadow of the U.S. armed forces, he said.

“We see them progressing rather dramatically across a variety of areas, but no, I don’t view them as 10-feet tall,” he said.