North Korea made threats on Thursday against South Korean commercial airliners that fly near its territory during U.S.-South Korean military drills next week, ratcheting up tensions with its neighbour.

"Security cannot be guaranteed for South Korean civil airplanes flying through the territorial air of our side and its vicinity ... above the East Sea of Korea (Sea of Japan) in particular, while the military exercises are under way," the North's KCNA news agency quoted a statement from a government official as saying.

Officials from South Korea's two major airlines, Korean Air and Asiana Airlines, said they would alter certain routes to move planes further away from North Korea, South Korea's Yonhap news agency said.

South Korea and the United States have held the military drills for years without major incident. North Korea regularly criticises the annual exercises as "a prelude to invasion and nuclear war." The drills will run from March 9-20.

In Washington, State Department spokesman Gordon Duguid called North Korea's statement "distinctly unhelpful" and said that instead of threatening civilian aviation, Pyongyang should be trying to comply with its commitments under stalled regional nuclear disarmament talks.

The prickly communist state has raised tensions in recent weeks by threatening to attack the South and by preparing to test-launch its longest-range missile.

The Taepodong-2 missile is designed to carry a weapon as far as Alaska, but has never flown successfully.

The North has said it is preparing to launch a satellite and has the right to do so as part of its peaceful space programme. North Korea is barred from test-firing its ballistic missiles under United Nations sanctions.

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

The United States, which led U.N. forces fighting for South Korea during the war, stations about 28,000 troops in the South to support Seoul's 670,000 soldiers. The impoverished North is one of the world's most militarised states, with some 1.2 million troops.

AID AND ANGER

North Korean generals will have military talks with the U.S.-led U.N. command on Friday, the U.N. command 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.

The United States has sent Stephen Bosworth, its new special envoy for North Korea, to the region this week for talks on halting any moves by Pyongyang viewed as provocative, and at having it return to sputtering nuclear disarmament discussions. Bosworth will visit Seoul on the weekend.

The aid-for-disarmament talks between the two Koreas, China, Japan, Russia and the United States on the North's nuclear weapons programmes have stalled, with Pyongyang refusing to agree to a request to set up a system to check its nuclear inventory and allow nuclear samples to be taken abroad.

North Korea has complained that aid is not being delivered as promised. Analysts do not expect it to launch a full-scale attack because its antiquated Soviet weaponry would be little match for the modern forces of the U.S.-backed South.

Pyongyang has also been angry at the policies of South Korean President Lee Myung-bak, who took office a year ago and cut off a free flow of unconditional aid. Lee instead tied Seoul's handouts to progress the North makes in reducing tension.