North Korea's military Tuesday threatened retaliation against the South over anti-North Korean protests as US Secretary of State John Kerry ended his Asian tour aimed at trying to defuse tensions.
Activists burn effigies of North Korea's late president Kim Il-Sung (L), current leader Kim Jong-Un (C) and late leader Kim Jong-Il (R) during an anti-Pyongyang rally in Seoul on Monday.
The warning came after protesters in Seoul on Monday burned portraits of North Korea's founder Kim Il-Sung, his son Kim Jong-Il and grandson and current leader Kim Jong-Un.
The act coincided with national celebrations in North Korea for the 101st anniversary of the birth of Kim Il-Sung amid expectations it would carry out a missile test to mark what it calls "The Day of the Sun".
"All the service personnel and people of the DPRK (North Korea) are simmering with towering resentment at this monstrous criminal act," the army's supreme command said in a release carried on the official Korean Central News Agency.
In an "ultimatum" to the South, it warned: "Our retaliatory action will start without any notice from now as such a thrice-cursed criminal act of hurting the dignity of the supreme leadership of the DPRK is being openly committed in the heart of Seoul under the patronage of the puppet authorities."
It added that the armed forces "will start immediately their just military actions to show how the service personnel and people of the DPRK value and protect the dignity of the supreme leadership.
"The military demonstration of the DPRK's revolutionary armed forces will be powerful sledge-hammer blows at all hostile forces hurting the dignity of the supreme leadership of the DPRK."
The fresh threat comes as the Korean peninsula has been in a state of heightened military tension since the North carried out its third nuclear test in February.
Incensed by fresh UN sanctions and joint South Korea-US military exercises, Pyongyang has spent weeks issuing blistering threats of missile strikes and nuclear war.
Top US diplomat Kerry on Monday wrapped up his tour of China, South Korea and Japan after pressing home the need for a united front against Pyongyang's erratic and bellicose behaviour.
Kerry had raised the prospect of "authentic" negotiations with Pyongyang if it took "meaningful steps" to show it would honour past commitments.
The White House said its position had long been that there was a path to talks for North Korea.
"It needs to be committed to the proposition of a denuclearised Korean peninsula," White House spokesman Jay Carney said.
Already assured of the support of US allies South Korea and Japan, Kerry said a commitment he received from China to work together to reduce tensions showed the world was speaking with one voice.
"One thing is certain: we are united. There can be no confusion on this point," he said in Tokyo Monday after meeting Prime Minister Shinzo Abe.
Washington insists that the "six-party" talks on denuclearisation -- which take in both Koreas, Japan, Russia, China and the United States -- is the only forum at which it will sit down with Pyongyang.
Kerry gave his blessing to peace moves by new South Korean President Park Geun-Hye, who has signalled the need to open a dialogue and "listen to what North Korea thinks".
But the North has rejected her proposals as a "crafty trick" to conceal Seoul's aggressive intentions.
North Korea's army supreme command Tuesday said that if South Korea really wanted dialogue and negotiations, "they should apologise for all anti-DPRK hostile acts, big and small".
The North has a habit of linking high-profile military tests with key dates, and expectations had been high of a mid-range missile test to coincide with Monday's celebrations marking the birth of Kim Il-Sung.
But unlike the centennial birth anniversary last year, there was no muscular military parade through the centre of Pyongyang and officials in Seoul said the "missile watch" could drag on for days.
The missiles mobilised by the North for a possible launch are reported to be untested Musudan models with an estimated range of up to 4,000 kilometres (2,485 miles).
That would cover any target in South Korea and Japan, and possibly even US military bases on the Pacific island of Guam.
South Korean and US forces have been on a heightened state of alert for days, and Japan has deployed Patriot anti-missile systems around Tokyo and promised to shoot down any missile deemed to be a threat.
South Korean Defence Ministry spokesman Kim Min-Seok said the alert would remain in force even in the absence of a missile launch on Monday.
"We believe the situation may drag on for quite a while," Kim said.