North Korea's Foreign Ministry said the country's military and the public stood ready to safeguard its regime even if it meant fighting an all-out war.

Pyongyang's official media said its military was not bluffing.

"The Korean People's Army (KPA) front-line large combined units entered into a wartime state all at once, fully armed to launch surprise operations, and wound up their preparedness for action," the North's official Korean Central News Agency was reported as saying by South Korean Yonhap News Agency.

The deputy North Korean ambassador to the United Nations, An Myong Hun, reiterated Pyongyang's threat of "strong military counter-action" if the South Korean broadcasts did not stop by 5:30 p.m. on Saturday, local time.

An also told reporters North Korea had asked the 15-member United Nations Security Council to hold an urgent meeting on the situation.

United Nations Secretary-General Ban Ki-moon appealed for the Koreas not to take any action that could further aggravate tensions, but diplomats said there had been no discussion among Security Council members about holding a meeting.

South Korea rejected the ultimatum that it halt anti-Pyongyang broadcasts by the Saturday evening deadline.

It began blasting anti-North propaganda over the Demilitarized Zone (DMZ) separating the countries on Aug. 10, resuming a tactic both sides had stopped in 2004, a few days after landmines wounded two South Korean soldiers along the DMZ.

North Korea resumed its own broadcasts on Monday and on Thursday, according to Seoul, launched four artillery shells into South Korea in apparent protest at loudspeaker broadcasts. The South fired back 29 artillery rounds. Pyongyang accused the South of inventing a pretext to fire into the North.

The United States, which has 28,500 military personnel based in South Korea, said it had resumed its annual joint military exercises there after a temporary halt to coordinate with Seoul over the shelling from North Korea.

The annual exercise, code-named Ulchi Freedom Guardian, began on Monday and runs until next Friday.

North Korea regularly condemns the maneuvers as a preparation for war and in the past has fired rockets into the sea while they proceeded.

South Korean Vice Defense Minister Baek Seung-joo said his government expected North Korea to fire at some of the 11 sites where Seoul has set up loudspeakers on its side of the DMZ.

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

In Washington, the U.S. assistant secretary of defense for East Asia, David Shear, said the United States was "very concerned" by the North Korean shelling and called on the North "to cease provocations across the DMZ and restore calm to the peninsula."

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 in clashes, yet the two sides have always pulled back from a return to all-out war.

Both sides reported no casualties or damage in their territory from the latest incidents, indicating that the exchanges this time were just warning shots.

Al Jazeera and wire services