A new Chinese fighter jet and a prototype Russian plane have similarities suggesting that Moscow may have passed on technology.
The fifth-generation J-20 fighter, which made its maiden flight in January in front of the visiting U.S. defense secretary, could have its origins in the Mikoyan 1.44 stealth jet that never made it to the production line.
Mikoyan technology may have been passed onto Chinese arms designers, a source close to Russia's defence industry said
"It looks like they got access to documents relating to the Mikoyan - the aircraft that the Ministry of Defence skipped over in its tender to create a stealth fighter," he said, speaking on condition of anonymity.
He said it was not clear whether such a transfer of technology had been legal.
Russia's help would allow it to keep tabs on China's rising military power's defence capabilities, experts have pointed out.
Independent analyst Adil Mukashev, who specialises in ties between Russia and China, suggested there had been a financial transaction.
"China bought the technology for parts, including the tail of the Mikoyan, for money," he said.
China's Defence Ministry declined a request for comment.
Russia's United Aircraft Corporation (UAC), which oversees production of the Mikoyan jets, denies any technology or design transfer took place with China.
Only the United States has an operational fifth-generation fighter, which is nearly impossible to track on radar.
Russia is working to start serial production of its prototype craft in the next five to six years.
China's creation of such a plane would put the country into an elite group of military powers, although analysts say it will take years to perfect the craft.
The source said Chinese officials had been invited to the plane's first public display when Russia was in the early stages of creating a fighter jet to compete with the U.S. F-22.
Rival designer Sukhoi was eventually contracted to help build the fighter and the Mikoyan 1.44, which lacks the radar-evading engineering of the U.S. F-22, was passed over.
Russia, the world's top energy producer, has fed China, the largest energy consumer, with natural gas and oil in its fast rise to become a global power.
But it has been unable to keep up with China's military spending, which was second only to the United States' in 2010.
Relations between the two countries are cordial but, in a sign that the two sides are suspicious of each other, Moscow is boosting its military capabilities in Russia's Far East to defend its position in resource-rich Siberia.
China, once a big buyer of Russian tanks, helicopters and jet fighters, has slowed its purchases from Moscow as its own production grew but military ties remain.
China's ambassador to Russia, Li Huei, was quoted last year as saying defence cooperation with Russia was moving beyond the buying and selling of weapons.
China is also trying to boost its naval power and its first aircraft carrier had its maiden voyage this month.
The re-fitted Soviet craft was bought from Ukraine.
"The Chinese aerospace industry is booming and developing rapidly," said Mikhail Pogosyan, head of UAC.
"In the aerospace industry what matters is the experience you have - not only to start a project but to see it through," he said.