Seoul  - South Korea on Friday condemned North Korea for "a military threat" and "an inhumane act," a day after Pyongyang said it could not guarantee the safety of South Korean civilian aircraft flying in or near its airspace.

"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," the Unification Ministry in Seoul said.

South Korea demanded that the North immediately withdraw its threat as South Korean airlines Korean Air and Asiana Airlines redirected their planes that had flown the affected routes.

Under an international agreement, South Korean airliners are permitted to travel briefly through North Korean airspace on routes between South Korean and US cities.

The indirect threat to shoot down South Korean planes was made as South Korea and the United States prepared to begin 11 days of annual joint military exercises Monday. Every year, the manoeuvres draw the wrath of Pyongyang, which charges they are a prelude to an invasion.

It also occurred amid rising tensions on the divided Korean Peninsula. North Korea has repeatedly threatened the South with destruction, accusing it of engaging in a confrontational inter-Korean policy.

South Korea has said that its Stalinist neighbour has been preparing for weeks to test-fire an intercontinental ballistic missile potentially capable of reaching the western US coast, but North Korea has said it is readying a satellite launch.

The US government also condemned North Korea's latest threat as "distinctly unhelpful."

Pyongyang should be focused on meeting its nuclear disarmament commitments under the six-nation talks "rather than making statements that are threatening to peaceful aviation," State Department spokesman Gordon Duguid said Thursday.

The United States and South Korea are involved in the six-nation talks with North Korea, China, Japan and Russia.

The talks have been stalled, however, over Pyongyang's refusal to allow samples to be taken at North Koean nuclear facilities as part of a procedure for ensuring it has fully disclosed the extent of its nuclear programme as part of the disarmament agreement.

North Korea blames the hold up on the refusal of the other countries to continue providing promised aid shipments.

North Korea's threat this week came as the new US envoy, Stephan Bosworth, was in the region for talks with China, Japan, Russia and South Korea. (dpa)