|A plane believed to be a Chinese J-20 stealth fighter is seen in Chengdu, Sichuan [Reuters - FILE PIC]
Balkan military officials have reported that China may have gleaned some of its technological knowledge for its recently tested stealth fighter from a US F-117 Nighthawk that was shot down over Serbia in 1999.
China's J-20 stealth fighter took its first test flight earlier this month, just days before a visit to Beijing by Robert Gates, the US defence secretary.
The Nighthawk, which belonged to the world's first ever range of stealth fighters, was shot down on March 27, 1999, during NATO's aerial bombing of Serbia in the Kosovo war.
"At the time, our intelligence reports told of Chinese agents crisscrossing the region where the F-117 disintegrated, buying up parts of the plane from local farmers," said Admiral Davor Domazet-Loso, Croatia's military chief of staff during the Kosovo war.
"We believe the Chinese used those materials to gain an insight into secret stealth technologies ... and to reverse-engineer them."
The downing of the Nighthawk, the pilot of which ejected and was rescued, was the first time one of the much-touted "invisible" fighters had ever been hit.
The Pentagon believed a combination of clever tactics and sheer luck allowed the Soviet-built SA-3 missile to bring down the fighter plane.
The wreckage was strewn over a wide area of flat farmlands, and civilians collected the parts, some the size of small cars, as souvenirs.
A senior Serbian military official confirmed that pieces of the wreckage were removed by souvenir collectors, and that some ended up "in the hands of foreign military attaches".
In Washington, an air force official, speaking on condition of anonymity, said the service was unaware of any connection between the downed F-117 plane and the development of Chinese stealth technology for the J-20.
Efforts to get comment from China's defence ministry were unsuccessful.
China's multi-role stealth fighter, known as the Chengdu J-20, made its inaugural flight on January 11, revealing dramatic progress in the country's efforts to develop cutting-edge military technologies.
Although the twin-engine J-20 is at least eight or nine years from entering action, it could become a rival to the top-of-the-line US F-22 Raptor, the successor to the Nighthawk and the only stealth fighter currently in service.
China rolled out the J-20 just days before the visit by Gates, leading some analysts to speculate that the timing was intended to demonstrate the growing might of China's armed forces.
Parts of the downed F-117 wreckage, such as the left wing with US Air Force insignia, the cockpit canopy, ejection seat, pilot's helmet and radio, are exhibited at Belgrade's aviation museum.
"I don't know what happened to the rest of the plane," said Zoran Milicevic, deputy director of the museum.
"A lot of delegations visited us in the past, including the Chinese, Russians and Americans ... but no one showed any interest in taking any part of the jet."
Zoran Kusovac, a Rome-based military consultant, said the government of Slobodan Milosevic, the late Serbian president, routinely shared captured Western equipment with its Chinese and Russian allies.
"The destroyed F-117 topped that wish-list for both the Russians and Chinese," Kusovac said.
Russia's Sukhoi T-50 prototype stealth fighter made its maiden flight last year and is due to enter service in about four years.
It is likely that the Russians also gleaned knowledge of stealth technology from the downed Nighthawk.
The F-117, developed in great secrecy in the 1970s, began service in 1983.
While not completely invisible to radar, its shape and radar-absorbent coating made detection extremely difficult.
The radar cross-section was further reduced because the wings' leading and trailing edges were composed of non-metallic honeycomb structures that do not reflect radar rays.
Kusovac said insight into this critical technology, and particularly the plane's secret radiation-absorbent exterior coating, would have significantly enhanced China's stealth knowledge.
Alexander Huang of Taipei's Tamkang University said the J-20 represented a major step forward for China.
He described Domazet-Loso's statement as "a logical assessment".
"There is no other stronger source for the origin of the J-20's stealthy technology," said Huang, an expert on China's air force.
"The argument the Croatian chief-of-staff makes is legitimate and cannot be ruled out."
Western diplomats have said China maintained an intelligence post in its Belgrade embassy during the Kosovo war.
The building was mistakenly struck by US bombers in May, 1999, killing three people inside.
"What that means is that the Serbs and Chinese would have been sharing their intelligence," said Alexander Neill, head of the Asia security programme at the Royal United Services Institute, a defence think tank in London.
"It's very likely that they shared the technology they recovered from the F-117, and it's very plausible that elements of the F-117 got to China."