SEOUL, South Korea - South Korean airlines are rerouting their flights away from North Korean airspace, hours after the North threatened Seoul's passenger planes amid heightened tensions on the divided peninsula.

The move -- which will cost carriers thousands of dollars on each flight -- comes a day after Pyongyang warned in state-run media that it cannot guarantee security for South Korean civil airplanes flying near its airspace and accused the U.S. and South Korea of attempting to provoke a nuclear war with the upcoming joint military drills.

It did not say what kind of danger South Korean planes would face or whether the threat meant the North would shoot down planes.

South Korea has urged the North to immediately retract the threat.

"The military threat against civil airplanes' normal flights is a violation of international norms and an inhumane act that cannot be justified under any circumstances," Unification said Ministry spokesman Kim Ho-nyeon.

He indicated the warning may be notice to clear the airspace before a possible missile launch but declined to elaborate.

North Korea announced last week that it is preparing to send a communications satellite into space, but regional powers suspect the claim is a cover for the launch of a long-range missile capable of reaching Alaska.

"We plan to make our flight detour through Japanese airspace until the crisis is resolved," said Park Hyun-soo, deputy general manager of Asiana Airlines' operations control center. He also said the rerouting will increase the flight time for about 40 minutes and cost about US$2,500) per flight.

Korean Air -- South Korea's biggest airline and the world's largest international cargo carrier -- has also taken similar steps to stay clear of the North.

In Washington, State Department spokesman Gordon Duguid said North Korea's statement was "distinctly unhelpful." He said North Korea should be working on ways to fulfill its disarmament commitments in international nuclear talks "rather than making statements that are threatening to peaceful aviation."

Also Friday, senior military officials from North Korea and the U.S.-led United Nations Command in South Korea met briefly at the border, UN command spokesman Kim Yong-kyu said without elaborating.

The North, which condemns upcoming military exercises with the South and the U.S. as preparations for an invasion, reportedly demanded that Washington call off the drills at previous talks earlier this week.

But the U.S. military said it would go ahead with the drills involving 26,000 U.S. troops, an unspecified number of South Korean soldiers and a U.S. aircraft carrier. Both Washington and Seoul insist the annual exercises are purely defensive.

The UN Command oversees the cease-fire that ended the 1950-1953 Korean War.