Pyongyang has asked the UN Security Council to hold an urgent meeting to discuss joint US-South Korean military exercises, the North Korean mission to the United Nations said Thursday.

North Korean Ambassador Ja Song Nam made the request in a letter to the council sent Wednesday, a day before Pyongyang and Seoul exchanged fire on their border.

North Korea has repeatedly asked the council to discuss the annual Ulchi Freedom military drill, which this year kicked off on Monday and will run until August 28.

The exercise is largely a computer simulation of a North Korean attack, but still involves 50,000 Korean and 30,000 US soldiers.

"Should the Security Council again ignore the DPRK's just request to discuss the US joint military exercise, it will expose of itself (sic) that it is giving up its primary mission of maintaining the international peace and security and becoming a political tool of an individual power," the ambassador wrote.

South and North Korea traded artillery fire across their border on Thursday in a rare exchange that sent tensions soaring to dangerous levels.

Asked about the exchange of fire, UN spokeswoman Eri Kaneko said "we are closely following the developments with serious concern, and we will revert as the situation evolves."