SEOUL (Reuters) - North Korea issued new threats against South Korea on Tuesday, vowing "sledge-hammer blows" of retaliation if South Korea did not apologise for anti-North Korean protests the previous day when the North was celebrating the birth of its founding leader.
The North also rejected what it called "cunning" U.S. overtures for talks, saying it will not be humiliated into being dragged to sit at the negotiating table by Washington.
But a senior U.S. military official in South Korea said the North Korean leadership was looking for a way to cool down its rhetoric after weeks of warnings of war.
On Monday, the North dropped its shrill threats against the United States and South Korea as it celebrated the 101st anniversary of the birth of its first leader, Kim Il-Sung, raising hopes for an easing of tension in a region that has for weeks seemed on the verge of conflict.
But the North's KCNA news agency said on Tuesday the North Korean army had issued an ultimatum to the South after rallies in the South on Monday at which portraits of North Korea's leaders were burned.
"Our retaliatory action will start without any notice from now," KCNA reported, citing military leaders of the Democratic People's Republic of Korea (DPRK), as North Korea is officially known.
The North's Foreign Ministry also rejected what it said was cunning U.S. scheming aimed at defusing tensions on the Korean peninsula with an offer of talks while deploying military assets capable of launching nuclear strikes against it.
"We do not oppose dialogue but we will not sit down at talks table in humiliation against opponents who are swinging the nuclear club against us," an unnamed Foreign Ministry spokesman said in comments carried by the KCNA news agency.
U.S. Secretary of State John Kerry said in Seoul last week that Washington was open to dialogue with Pyongyang on the condition that the talks would lead to eliminating nuclear arsenal from the North.
South Korean media reported several small demonstrations in the capital, Seoul, on Monday. One television station showed pictures of a handful of protesters burning a portrait of North Korean leader Kim Jong-un.
Small counter-protests, by South Koreans calling for dialogue with the North, were also held, media reported.
The North has threatened nuclear attacks on the United States, South Korea and Japan after new U.N. sanctions were imposed in response to its latest nuclear arms test in February.
The North has also been angry about annual military exercises between U.S. and South Korean forces, describing them as a "hostile" act. The United States dispatched B52 and B2 stealth bombers from their bases to take part.
But along with the new threat on Tuesday, the North's KCNA raised the possibility of dialogue.
"If the puppet authorities truly want dialogue and negotiations, they should apologise for all anti-DPRK hostile acts, big and small, and show the compatriots their will to stop all these acts," KCNA cited the North's military as saying.
A South Korean Foreign Ministry spokesman later told a briefing the North Korean ultimatum was not worth a response and South Korea was waiting for the North to make a "wise decision".
Last week, the South's President Park Geun-hye offered talks but the North rejected the overture as a "cunning" ploy.
Park will meet U.S. President Barack Obama at the White House on May 7 to discuss economic and security issues, including "countering the North Korean threat", the White House said on Monday.
The U.S. military official, who spoke on condition of anonymity, said a North Korean missile test or nuclear test were possible but he believed it was trying to tone down its the war of words.
"The DPRK leadership is trying to figure out a way to off-ramp from the heightened state of rhetoric that we've been seeing for the past several weeks," the official told reporters.
North Korea faced difficulties trying to "fix and tune up" its Soviet-era conventional weapons, and that was why it wanted nuclear weapons, and the missiles to deliver them.
"They are replacing that decreasing conventional capability with increasing asymmetric capability of weapons of mass destruction, intercontinental ballistic missiles and special operations forces," the official said.
The United States has offered talks with the North, but on the pre-condition that it abandons its nuclear weapons ambitions. North Korea deems its nuclear arms a "treasured sword" and has vowed never to give them up.
However, U.S. Secretary of State Kerry, ending his visit to Korea, appeared to open the door to talking without requiring the North to take denuclearisation steps in advance. Beijing, he said, could be an intermediary.
North Korea has conducted three nuclear tests but it was not believed to be near weapons capability.
Missile launches and nuclear tests by North Korea are both banned under U.N. Security Council resolutions that were expanded after the North's February test.
The aim of the North's aggression, analysts say, is to bolster the leadership of Kim Jong-un, the 30-year-old grandson Kim Il-Sung, or to force the United States, which has 28,000 troops in South Korea, to open talks.
A U.S. Marine transport helicopter crashed in South Korea on Tuesday, near the border with North Korea, with 21 people on board during exercises with South Korean forces.
The U.S. military described the accident as a "hard landing" and said six people were in stable condition in hospital. South Korean media said the helicopter caught fire after all on board got out. The cause of the accident would be investigated, the U.S. military said.
Additional reporting by Se Young Lee and Christine Kim; Editing by Paul Tait and Michael Perry