The Democratic People's Republic of Korea (DPRK) Wednesday abandoned a truce that ended the Korean War and threatened to attack those who had provoked the country, in response to Seoul's decision to join a US-led international security initiative.
Pyongyang was also restarting a plant capable of making plutonium for nuclear bombs, two days after it tested an atomic bomb for the second time since 2006, Republic of Korea (ROK) media reported.
The moves added to mounting tension in the region this week, during which the DPRK also test-fired at least five short-range missiles.
"Any hostile act against us including the stopping and searching of our peaceful vessels will be considered an unpardonable infringement on our sovereignty and we will immediately respond with a powerful military strike," a statement on Pyongyang's official Korean Central News Agency said.
The warning invalidated the July 27, 1953 armistice and said Washington was "indulging the ROK" to participate in the Proliferation Security Initiative (PSI), which had "dragged the Korean Peninsula into a state of war".
Pyongyang also said it would not guarantee the safety of US and ROK warships, as well as civilian vessels, in the region.
"Those who have provoked us will face unimaginable, merciless punishment," the DPRK statement said.
The PSI, which was established by Washington after the Sept 11 attacks in 2001, includes more than 90 nations and provides for the stopping of vessels to ensure that they are not carrying weapons of mass destruction or components for making them. Seoul announced it was joining the initiative on Tuesday.
Seoul's mass-circulation Chosun Ilbo newspaper also reported Wednesday that US spy planes had detected signs of steam at the Yongbyon nuclear plant, an indication that the facility may have started reprocessing nuclear fuel.
Pyongyang has enough plutonium for at least half a dozen atomic bombs, AP quoted an unnamed source as saying.
Still, Chinese analysts Wednesday downplayed the likelihood of a full-scale conflict between the two sides of the Korean Peninsula.
Zhang Minqian of the Beijing-based University of International Relations said the DPRK was "deterring its adversaries" by abandoning the armistice.
"But the possibility of war is not high, as the DPRK knows it will have to face the ROK-US alliance for that," Zhang said, adding that the difference in power between the two groups was "enormous".
Shi Yinhong, who specializes in international security at the Renmin University of China, also said the chance of any large-scale conflict in the region was small. "But the latest moves may lead to minor clashes in some regions between the DPRK and the ROK."
The ROK defense ministry said no reinforcements were being sent to the region, AFP reported. A US Treasury Department official also told Reuters it was weighing possible action to isolate the DPRK financially.
Agencies contributed to the story