Senior military officials from North Korea and the U.S.-led U.N. Command in South Korea opened talks at the border Friday, a day after the North threatened passenger planes flying near its airspace.

The talks — the second such talks between the two sides this week — come ahead of U.S.-South Korean military drills and amid concerns the North is preparing to test-fire a long-range ballistic missile.

The North, which condemns the drills as preparations for an invasion, reportedly demanded that Washington call off the exercises 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.

Stepping up the rhetoric on the eve of the second meeting, North Korea warned 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.

Generals from the North summoned U.S. military officers representing the U.N. Command to the Demilitarized Zone on Monday — their first talks in nearly seven years — but the talks ended without progress. The U.N. Command oversees the cease-fire that ended the 1950-1953 Korean War.

The North is "compelled to declare that security cannot be guaranteed for South Korean civil airplanes flying through the territorial air of our side and its vicinity ... while the military exercises are under way," the North's Committee for the Peaceful Reunification of the Fatherland said Thursday in statement carried by the state-run Korean Central News Agency.

It did not say what kind of danger South Korean planes would face or whether the threat means the North would shoot down planes. It was also unclear whether the statement was a warning to clear the air before a possible missile launch.

In response, South Korea's Korean Air and Asiana Airlines rerouted their flights to stay clear of North Korean airspace.

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."

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.