North Korea, which is preparing to test its longest-range missile, said it could not guarantee the safety of the South's commercial flights off the east coast of the peninsula where the missile base is located.
It linked the warning to next week's joint U.S.-South Korea military drills, which it has routinely attacked as an attempt to provoke nuclear war.
Caution: Korean Air jets have been rerouted following the threat from Pyongyang
South Korea's Korean Air and Asiana Airlines have rerouted their flights.
Pyongyang is 'compelled to declare that security cannot be guaranteed for South Korean civil airplanes flying through the territorial air of our side and its vicinity ... while the military exercises are under way,' a statement said.
The U.S. called the statement 'distinctly unhelpful' while South Korea demanded it be withdrawn.
A government spokesman in Seoul warned: 'Threatening civilian airliners' normal operations under international aviation regulations is not only against the international rules but is an act against humanity.'
Observers said the threat by Kim Jong-Il's hardline regime was aimed at escalating tensions and that planes were actually in little danger.
