Shanghai, with ambitions to become a global transport and business hub, mysteriously closed its international airport for two hours on Friday, apparently because of a military exercise.

Outbound flights were delayed while flights into the east coast city were diverted to other airports, said an official at Shanghai Pudong Airport's air command center. 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.

Asked the reason, the official said the closure was on the orders of the Central Military Commission, adding: "It is a state secret." He did not give his name.

A passenger flight and a cargo flight from Japan Airlines (JAL) left Japan for Shanghai around noon on Friday but had to turn back after notice that China was conducting a military exercise, a spokesman for Tokyo's Narita Airport said.

The spokesman could not say if this was the first such case but added, "Usually, we are given advanced notice when countries are conducting a military exercise."

Hong Kong media also said the closure was due to a military exercise.

A Shanghai city government spokeswoman confirmed the closure, but gave no reason for it.

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