SEOUL (Reuters) - South Korea told the North on Friday to immediately withdraw a threat it made against the South's commercial airliners, which has forced them to stop flying near the airspace of the reclusive communist state.

Singapore Airlines, the world's biggest by market value, said it was joining South Korean carriers in avoiding North Korean airspace as a precaution, although other regional carriers were not altering their flight paths.

North Korea, which is preparing to test its longest-range Taepodong-2 missile, said on Thursday 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.

North Korea linked its warning to joint U.S.-South Korean military drills, which start on Monday and have been held for years without major incident. The prickly North regularly criticizes them as a prelude to invasion and war.

The U.S. special envoy for North Korea, Stephen Bosworth, criticized Pyongyang for the aircraft threat.

This is, we believe, very undesirable. It's provocation and it's unacceptable, Bosworth told reporters in Tokyo during a visit to the region for talks aimed at coaxing North Korea back to faltering nuclear disarmament negotiations.

South Korea's Unification Ministry spokesman Kim Ho-nyeon said the South's Korean Air and Asiana Airlines had been immediately notified of the threat. The airliners responded by diverting flights that approach the country from the east, he said.

Kim said about 33 daily flights approached the South from the east with about 15 of them by South Korean airliners.

Threatening civilian airliners' normal operations under international aviation regulations is not only against the international rules but is an act against humanity, he said.

The government urges the North to immediately withdraw the military threat against civilian airliners.

MISSILE LAUNCH

Singapore Airlines said it was avoiding North Korean airspace and using alternative routes, but added in an email to Reuters the move would not significantly affect flight times.

Japan Airlines, All Nippon Airways and Air China said they had no plans to alter their flight paths.

The area would likely be in the flight path of the missile, which spy satellites indicate is still in an assembly facility. It takes North Korea at least a week to prepare the missile for flight after setting it vertically and moving it to a launch pad, experts have said.

We've become quite used to our northern neighbor's threats. Its overall impact on airlines is limited, said Suh Jin-hee, an analyst at SK Securities in Seoul.

North Korea on Friday repeated it was preparing to launch a satellite as part of its peaceful space program.

South Korean officials said they saw no difference between a satellite and missile launch because they use the same technology and the same rocket. The North is barred from test-firing its ballistic missiles under United Nations sanctions.

MILITARY TALKS

The U.S.-led United Nations command also urged North Korea not to take any provocative actions.

The comments came after North Korean generals met a U.N. delegation for 45 minutes of talks on military issues at the Panmunjom truce village in the Demilitarised Zone, a buffer that separates the two Koreas.

The U.N. command told officers from the Korean People's Army (KPA) that the threat was entirely inappropriate ... and should be retracted immediately, the U.N. command said in a statement.

The head of the North's delegation said the planned satellite launch was an entirely just measure for self-defense, according to a report by the North's KCNA news agency.

He strongly warned the U.S. forces side that as long as it does not cancel the DPRK-targeted war exercises, the KPA will take strong countermeasures to cope with the policy taken by the new U.S. administration, KCNA said.

The two sides had their first such meeting in about seven years on Monday and the North, which requested the talks, complained about U.S. military moves near the border and live-fire joint training, South Korean officials said.

U.S. envoy Bosworth, speaking after meeting Japanese officials, said the two nations would like to restart six-party talks, also involving China, Russia and the two Koreas, on ending the North's nuclear arms program as soon as possible.

We agreed strongly that it would be best if North Korea did not fire a missile, whether it's a satellite launch or missile, for us it makes no difference, he said.

The two Koreas are technically still at war and station about 1 million troops near their respective sides of the Demilitarised Zone that has divided the peninsula since the 1950-53 Korean War ended in a ceasefire, but not a peace treaty.

(Additional reporting by Park Jung-youn and Yoko Nishikawa in Tokyo; Editing by Jon Herskovitz and Alex Richardson)

South Korea