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The US is playing down pictures that appear to show a working prototype of a Chinese stealth aircraft, invisible to radar.

The images - first published on websites - show what looks like a stealth fighter on a taxi run.

Beijing has not commented on whether the pictures - published ahead of US Defence Secretary Robert Gates' visit later this week - are genuine.

The Pentagon says China is still years away from deploying a stealth aircraft.

In late 2009 the deputy head of China's air force, General He Weirong, said the country's stealth fighter would be ready sometime between 2017 and 2019, reports said.

But US director of naval intelligence Vice Admiral David Dorsett said that it would be "years" before China's new fighter would be operational.

"Developing a stealth capability with a prototype and then integrating that into a combat environment is going to take some time," he said.

The leaked photos of the prototype aircraft first appeared on military websites and blogs. They were said to have been taken at the Chengdu Aircraft Design Institute.

The images were then discussed in the Chinese state-run Global Times newspaper, in both its Chinese and English-language editions, although no comment was made on their authenticity.

Japan's Asahi Shimbun newspaper said China could begin test flights of the stealth jet as soon as this month, citing unidentified Chinese military sources.

The world's only operational stealth fighter is the Lockheed Martin F-22 Raptor, but Russia is also known to be working on its own prototype.

´óÏó´«Ã½ defence correspondent Nick Childs says the aircraft certainly bears a superficial resemblance to the latest US stealth designs - and that may spook some in Washington.

And it will add to the concerns of those who have been warning especially of the increasing reach and capabilities of China's air force and navy, he adds.

The US has been watching closely as China increases its military capacity - in particular, its development of a so-called "carrier killer" missile, a land-based system which could sink an aircraft carrier from up to 1,800 miles (2,900km) away.

US battle groups - including aircraft carriers - are stationed in the South China Sea.

And in August, the Pentagon reported that China had been expanding its nuclear arsenal and submarine force, as well as upgrading its land-based missiles.