North Korea is asking the UN Security Council to hold an urgent session to discuss upcoming US-South Korean joint military exercises that Pyongyang described on Friday as a threat to peace.

Deputy representative Ri Tong Il told reporters that the top world body had not responded to the request contained in a July 21 letter, and vowed North Korea will keep up missile launches in response to the planned war games.

"They are disregarding us. We cannot accept this," Ri told reporters.

"If the UN Security Council turns away from this request for an emergency meeting, it will only expose itself as a UN body that has lost its principles, lost impartiality and lost its mandate of peace and security."

The annual Ulchi-Freedom Guardian exercise, aimed at testing combat readiness for a North Korean invasion, is scheduled to begin August 18.

Although largely played out on computers, it involves tens of thousands of South Korean and US troops.

North Korea's UN envoy said his country would keep up rocket and artillery launches, which he described as "more than justifiable" to counter the threat posed by the joint exercises.

"Ongoing tactical rocket launches of all different types of artillery fire are being conducted by the DPRK," said Ri.

"It is quite natural, more than justifiable, because this is in response to the grave situation created by the large-scale joint military exercises," he said. "The war danger is being increased."

On Wednesday, North Korea fired four short-range projectiles into the sea, in the latest of a series of missile, rocket and artillery tests that the UN Security Council has condemned as illegal.

"All these kinds of rocket launches are giving great strength and encouragement to the Korean people's army and the people so that they can make steadfast progress in nation-building," said Ri.

North Korea has always protested against the staging of joint military drills in the South, but usually to little avail.

South Korea's defense ministry rejected the complaint and said the Ulchi exercise would go ahead as planned.

"North Korea's military threats and rhetoric are going too far these days," ministry spokesman Wee Yong-Sub said.