Shanghai, which aims to become a global transport and business hub, mysteriously closed its international airport for two hours on December 1.

Outbound flights were delayed while flights into Shanghai were diverted to other airports, said an official at Shanghai Pudong International Airport's air command centre. The closure was not publicly advertised ahead of time.

"It was closed for two hours. The airspace control was ordered by the state. It is not the airport's problem," the official said by telephone.

Asked the reason, the official replied: "It is a state secret." He did not give his name, but a city government spokeswoman confirmed the closure.

China frequently delays or cuts flights because of bad weather, and sometimes closes airports temporarily for security reasons if top officials are passing through. But major restrictions at Pudong International Airport, serving the country's biggest city, are rare.