Washington  - North Korea's threats Thursday against South Korean airlines that fly into its airspace are "distinctly unhelpful," the US State Department said.

The North Korean government issued a statement saying it cannot guarantee the safety of South Korean civilian airlines flying near or in its airspace over the Sea of Japan while the United States and South Korea conduct joint military exercises.

"They are distinctly unhelpful," State Department acting deputy spokesman Gordon Duguid said.

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," Duguid said.

The United States and South Korea are set to begin 11 days of annual exercises on Monday. Pyongyang has criticized the exercises as a prelude to an invasion.

North Korea's warning Thursday is the most recent of a series of threats. Pyongyang has threatened to destroy South Korea and is preparing to test an intercontinental ballistic missile potentially capable of reaching the western US coast. Washington has urged North Korea to abandon the "provocative" plans.

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 six-nation agreement with North Korea has been stalled over Pyongyang's refusal to allow nuclear samples to be taken 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 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, the four other nations in the six-party negotiations.

Bosworth has no plans to meet with North Korean officials, Duguid said. The veteran diplomat was in Tokyo Thursday and is due in Seoul this weekend. (dpa)