North Korea's deputy United Nations ambassador says "a strong military counteraction will be launched" if South Korea does not meet a Saturday deadline for removing loudspeakers broadcasting anti-North Korean propaganda along their border.

"I would say that our military will not make any empty talks," An Myong Hun, North Korea's deputy UN ambassador, told reporters.

An says North Korea has asked for an urgent meeting of the UN Security Council to discuss issues including the current U.S.-South Korea military exercises, which were briefly suspended.

The letter to the UN Security Council president accuses South Korea and the U.S. of "fabricating" the land mine incident that led to the current escalation of tensions.

Meanwhile, UN Secretary General Ban Ki-moon has appealed for North Korea and South Korea not to take any action that could further escalate tensions after Seoul refused to halt propaganda broadcasts and Pyongyang put its troops on a war footing.

South Korean Vice Defence Minister Baek Seung-joo said on Friday it was likely the North would fire at some of the 11 sites where the loudspeakers are set up on the South's side of the demilitarized zone (DMZ) separating the two countries.

United Nations Secretary General Ban Ki-moon has urged North and South Korea not to take any actions that might increase tensions. (AFP/Getty Images)

On Thursday, North Korea fired four shells into South Korea, according to Seoul, in apparent protest against the broadcasts. The South fired back 29 artillery shells. Pyongyang accused the South of inventing a pretext to fire into the North.

North Korean Leader Kim Jong-un has declared a "quasi-state of war" in front-line areas, according to the North's official KCNA news agency

Both sides said there were no casualties or damage in their territory, an indication that the rounds were just warning shots.

"The fact that both sides' shells didn't damage anything means they did not want to spread an armed clash. There is always a chance for war, but that chance is very, very low," said Yang Moo-jin, professor at the University of North Korean Studies in Seoul.

Ban, a former South Korean foreign minister, was deeply concerned by the latest developments, said UN spokeswoman Eri Kaneko.

"He urges the parties to refrain from taking any further measures that might increase tensions," Kaneko said. "He also calls on parties to engage in dialogue to reduce tensions and to promote peace and stability on the Korean Peninsula."

China calls for calm

China, which remains North Korea's main economic backer despite diminished political clout to influence Pyongyang, said on Friday it was deeply concerned about the escalation of tension and called for calm from both sides.

Since the 1950-53 Korean War ended in a truce, not a peace treaty, Pyongyang and Seoul have often exchanged threats, and dozens of soldiers have been killed, yet the two sides have always pulled back from all-out war.

A North Korean official has asked for an urgent meeting of the UN Security Council to discuss issues including the current U.S.-South Korea military exercises. (REUTERS) But the renewed hostility is a further blow to South Korean President Park Geun-hye's efforts to improve North-South ties, which have been virtually frozen since the deadly 2010 sinking of a South Korean navy ship, which Seoul blames on Pyongyang.

Park cancelled an event on Friday and made a visit to a military command post, dressed in army camouflage.

Both sides traded harsh rhetoric late on Friday.

The North committed "cowardly criminal acts," South Korean Defence Minister Han Min-koo said. "This time, I will make sure to sever the vicious cycle of North Korea's provocations."

The North's official KCNA news agency said its military was not bluffing.

Deadline set for Saturday

The North's shelling came after it had demanded last weekend that South Korea end the broadcasts or face military action — a relatively rare case of following up on its frequent threats against the South.

Its 48-hour ultimatum, delivered in a letter to the South Korean Defence Ministry, was also uncharacteristically specific, said John Delury, a North Korea expert at Yonsei University in Seoul. The deadline is at 5 p.m. local time (4 a.m. ET) on Saturday in Seoul.

South Korea began blasting propaganda messages from loudspeakers on the border on Aug. 10, resuming a tactic both sides had stopped in 2004, days after land mines wounded two South Korean soldiers along the DMZ.

North Korea began its own broadcasts on Monday.

Baek told parliament the South's broadcasts would continue unless the North accepted responsibility and apologized for the mines. Pyongyang has denied responsibility.

The U.S. military, which bases 28,500 personnel in South Korea, said it was monitoring the situation.