Airlines in South Korea have diverted flights after North Korea said their safety could not be guaranteed.
The danger to flights comes as tensions rise between North and South Korea.
In recent weeks the North has threatened to attack the South and has been a preparing a long-range missile for testing.
Around 33 flights a day to South Korea pass through airspace likely to be in the path of a North Korean missile test.
The two Koreas are technically still at war and station about a million troops near their respective sides of the Demilitarised Zone, a tense border which has divided the peninsula since 1953.
Tensions were given added momentum as South Korea and the United States carry out one of their regular joint training missions.
The North has described the US and South Korea as imperialistic and warlike lunatics who are bent on attacking North Korea.
The military posturing and verbal attacks are standard fare in nearly 60 years of antagonism between North and South Korea, a region where the smallest incident can soon escalate.
For its part,the North is preparing to test a Taepodong 2 missile which it says is capable of reaching the US state of Alaska.