China denies cyber attack allegations
A secretive branch of China's military is probably one of the world's "most prolific cyber espionage groups", a US cyber security firm has said.
Mandiant said Unit 61398 was believed to have "systematically stolen hundreds of terabytes of data" from at least 141 organisations around the world.
The firm tracks hundreds of cyber-spying cases around the world.
However, the Chinese government has vigorously denied the allegations.
The BBC's John Sudworth reports from Shanghai.