More than 20 countries have established air defense identification zones. They were first defined in the 1950s after World War II, Naval Military Academic Institute researcher Zhang Junshe said.

The United States asserted control of coastal airspace when the war broke out. After the 1941 Japanese attack on Pearl Harbor, the US felt a more urgent need to control coastal airspace.

With the 1950 outbreak of the Korean War, the US Air Force placed its forces on alert and discovered major weaknesses in domestic defense units' coordination.

That year, the US developed an air defense command and control structure, and established five air defense identification zones around North America.

The US formally defines its ADIZ in the Code of Federal Regulations. It refers to an airspace surrounding the US and Canada.

This ADIZ is jointly administered by both countries' civilian air traffic control authorities and militaries under the North American Aerospace Defense Command.

"No person may operate an aircraft into, within or from a departure point within an ADIZ, unless the person files, activates and closes a flight plan with the appropriate aeronautical facility or is otherwise authorized by air traffic control," the code said.

Other countries, including Japan, South Korea and Australia, have ADIZs.

- Zhou Wa