Pyongyang's armed forces have threatened South Korea with an imminent attack unless "anti-North Korean" activities stop. Meanwhile, a US military helicopter has crashed near the North Korean border.
The Korean People's Army demanded an apology from Seoul on Tuesday a day after a South Korean conservative group burnt effigies of North Korea's leader Kim Jong Un and former leaders Kim Jong Il and Kim Il Sung during a demonstration.
In a statement carried by the North's official news agency, the Korean Central News Agency (KCNA), the military warned of retaliation if similar protests were repeated.
"The supreme command of the Korean People's Army Tuesday issued an ultimatum to the South Korean puppet group," the KCNA reported.
Condemning what it described as a "thrice-cursed... monstrous criminal act," it warned of "powerful sledge-hammer blows at all hostile forces hurting the dignity of the supreme leadership,"
South Korea's Defense Ministry spokesman Kim Min Seok described the North's latest threat as "regrettable," warning of retaliation against any military provocation.
"We will retaliate thoroughly and resolutely to provocations staged for any reasons," the spokesman told reporters in Seoul.
South Korean President Park Geun-hye is scheduled to meet with US President Barack Obama at the White House on May 7 to coordinate a response to North Korean aggression.
US helicopter crash
Meanwhile, a US military helicopter crashed close to North Korean territory on Tuesday, according to the Yonhap news agency, quoting defense sources. The accident occurred during a joint US-South Korean military exercise.
None of the crew members or 13 US soldiers on board were injured.
Initial reports did not indicate the cause of the crash.
Monday's protest coincided with the 101st anniversary of the birth of North Korea's late founding leader Kim Il Sung, the grandfather of the current leader Kim Jong Un and father of Kim Jong Il.
It also came at a time of heightened nuclear rhetoric in the Korean Peninsula. Pyongyang has warned of attacks against the US, South Korea and Japan in recent weeks after the UN imposed new sanctions in response to its third nuclear test in February.
US Secretary of State John Kerry ended a four-day Northeast Asian tour aimed at defusing escalating tensions on Monday. Speaking in Tokyo Kerry said the US was open to negotiations with North Korea over its nuclear policy, provided it took "meaningful steps" towards peace.
"The United States remains open to authentic and credible negotiations on denuclearization, but the burden is on Pyongyang," Kerry said. "North Korea must take meaningful steps to show it will honor commitments it has already made."
ccp/av (AFP, AP, dpa)