The latest threat to attack South Korea came in a radio broadcast from Pyongyang in the name of the "supreme authority" of North Korea's army.
In a statement read on air Tuesday, North Korea's military threatened unspecified "immediate" action against the South if it did not apologize for the small Monday protests, during which effigies of North Korean leaders were burned.
Conservative groups in South Korea frequently stage such small-scale rallies, but this latest one took place on the birth anniversary of North Korea's founder Kim Il Sung, who is still revered there as the country's eternal president.
Related - North Korea Marks Birth of Founder Amid Tensions
South Korean officials bristled at North Korea's contention its ultimatum is justified by the action of the activists in Seoul.
Cho Tai-young, a spokesman for the Ministry of Foreign Affairs, said the North's argument is illogical and has no merit. He said the South hopes the North will make a wise and correct choice.
South Korea's Defense Ministry said it is closely monitoring North Korean military movements and maintaining a firm readiness posture. And, if North Korea provokes for any reasons, a defense spokesman warns, there will be a thorough and resolute retaliation.
North Korea has issued a series of threats in recent weeks, promising war. It has also vowed to continue with its development of nuclear weapons and ballistic missiles in defiance of various United Nations sanctions.
Meanwhile, a U.S. military helicopter executed what officials call a “hard landing” at a shooting range on routine flight operations near the border with North Korea.
The helicopter caught fire after all on board evacuated.
A media release from U.S. Forces Korea says all 21 personnel on the helicopter, including the five crew members from the 31st Marine Expeditionary Unit out of Okinawa, Japan, were taken to a military hospital in Seoul.
The marines were participating in a joint exercise with South Korean forces.
The CH-53E Sea Stallion they were aboard is the largest and heaviest helicopter flown by the U.S. military.
In Washington, U.S. President Barack Obama said he expects more posturing and "provocative moves" from North Korea over the next several weeks.
He told NBC's Today
that while his administration does not believe Pyongyang has the capacity to put a nuclear weapon on a ballistic missile, the U.S. is preparing for "every contingency."
The threat comes as North Koreans continue their two-day celebration of the birthday of late founding leader Kim Il Sung. Many had expected Pyongyang to mark the occasion with a provocative missile test, but the Monday anniversary passed without incident.