Seoul, South Korea ? Air Canada and Singapore Airlines joined South Korean airlines in rerouting flights to steer clear of North Korean airspace Friday after the communist regime threatened Seoul’s passenger planes amid heightened tensions on the Korean peninsula.

North Korea warned late Thursday that it cannot guarantee the safety of South Korea’s passenger jets flying near its airspace if annual joint U.S.-South Korean military maneuvers go ahead as planned Monday.

South Korea’s two main airlines, Korean Air and Asiana Airlines, immediately began redirecting flights away from the North’s airspace.

On Friday, at least two foreign airlines, Air Canada and Singapore Airlines, also changed flight paths to and from Seoul, an official at the Civil Aviation Safety Authority said. He agreed to discuss the matter only if not quoted by name because he was not authorized to talk with journalists.

The two American carriers that fly to South Korea, Delta Air Lines Inc. and UAL Corp., said their routes did not take their planes near North Korea and they had not canceled or rerouted flights.

Pyongyang’s warning was the latest threat from North Korea at a time of mounting tensions over stalled reconciliation efforts and the North’s plan for a missile test. The two Koreas technically remain at war because their bitter 1950-53 war ended in a cease-fire rather than a peace treaty.

Relations have worsened since conservative South Korean President Lee Myung-bak took office a year ago and refused to provide the impoverished North with aid unless the regime abided by its commitment to dismantle its nuclear program.

North Korea cut off ties, canceled joint inter-Korean projects and declared peacekeeping agreements with the South void.

Last year, the North stopped disabling its nuclear program, and last week it announced it was preparing to send a communications satellite into space — a launch that nations in the region suspect is a cover for testing a long-range missile capable of reaching Alaska.

In issuing its threat to South Korean airliners, the North did not say what kind of danger jetliners might face. It was not clear whether the North was threatening to shoot down planes.

Korean Air, South Korea’s largest airline, has twice had planes downed: one shot down in 1983 by a Soviet fighter jet with the loss of all 269 people aboard and another destroyed by a bomb allegedly planted by North Korean agents in 1987 that killed all 115 people on board.

South Korea urged the North to 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 Ministry spokesman Kim Ho-nyeon told reporters Friday.

Kim hinted the threat could be a way to clear airspace before a possible missile launch.

U.S. generals representing the U.N. Command, the American-led body overseeing the cease-fire between the two Koreas, told their North Korean counterparts Friday that the threat was “inappropriate.” They urged the North to retract the warning, the U.N. Command said.

North Korean generals rejected the demand, calling the warning a “self-defense measure,” according to Pyongyang’s Korean Central News Agency. The North’s chief delegate at the meeting warned of “strong countermeasures” unless the U.S. called off the military exercises with South Korea, and he reasserted Pyongyang’s right to launch a satellite into space, KCNA said.

In Washington, State Department spokesman Gordon Duguid criticized the threat. “North Korea’s belligerent rhetoric is unwarranted and counterproductive,” he said.