In a statement late on Thursday North Korea blamed joint military exercises by the US and South Korea due to begin next week, saying it was "compelled to declare that security cannot be guaranteed for South Korean civil airplanes ... while the military exercises are under way."
"A military threat to the normal operations of civil airplanes not only violates international rules but is also an inhumane act that can never be justified"
South Korean unification ministry statement
No one knows "what military conflicts will be touched off by the reckless war exercises" south of the border, it said.
Following the announcement South Korean carriers Korean Airlines and Asiana have both said early on Friday that they would reroute flights well away from North Korean airspace as a safety measure.
"All planes will be directed to fly further south over the Pacific Ocean when heading for or arriving from North America," South Korea's Yonhap news agency quoted a Korean Airlines spokesperson as saying.
Diverting flights away from North Korean airspace will add about an hour extra flying time to routes to and from North America.
On Friday generals from North Korea and the US-led United Nations Command are due to hold follow-up talks at the border truce village of Panmunjom after meeting on Monday for the first time in almost seven years.
The UN Command oversees the fragile ceasefire along the so-called Demilitarised Zone dividing North and South Korea, which was established at the end of the 1950-53 Korean War.
'Provoking nuclear war'
During Monday's meeting, which lasted about half an hour, North Korea demanded the US and South Korea call off next week's exercises, which it said was an effort to provoke a nuclear war.
| North Korea has repeatedly accused the South of plotting to invade the country [AFP]
On average 10 daily flights by South Korean airlines fly over an area of airspace administered by North Korea over the Sea of Japan, or East Sea.
The air corridor which passes over Russia's Kamchatka peninsula is used to link Seoul's Incheon airport with US cities like New York, Los Angeles and Chicago and some Russian destinations.
North Korea receives about $500 for every flight using the corridor.
The route has sensitive memories for South Koreans.
In 1983, at the height of Cold War tensions, a Korean Airlines Boeing 747 was shot down by Soviet fighter jets as it flew close the Kamchatka peninsula, killing all 269 on board.
Yonhap quoted military sources as saying the North Korean warning may be a sign that it is trying to clear its airspace ahead of the suspected ballistic missile test.