North Korea on Friday threatened "physical countermeasures" if Seoul participates in sanctions over its nuclear weapons program.
It was the latest in a string of escalatory rhetoric aimed at regional players after the U.N. Security Council this week adopted resolution 2087 in response to Pyongyang's Dec 12 long-range rocket launch.
"If the South takes direct part in the U.N. sanctions, the (North) will take strong physical countermeasures against it," the Committee for Peaceful Reunification of Fatherland said in a statement carried by the official Korean Central News Agency. It added, "Sanctions amount to a declaration of war."
Unification Ministry deputy spokeswoman Park Soo-jin said the North should refrain from making such "inappropriate threats."
"North Korea should refrain from making further provocations, comply with U.N. resolutions and take the right path of cooperation with the international community," she said.
On Thursday, Pyongyang vowed to continue its missile tests and conduct a "higher-level" nuclear test, saying such moves aimed at foiling Washington's "hostile policy" toward it.
That statement also swiped at Beijing for joining the United States in adopting the resolution, which adds entities and individuals to a U.N. sanctions list. The resolution warns of "significant action," if Pyongyang carries out nuclear test.
The latest bellicosity set back hopes for improved atmosphere in Northeast Asia under new leaderships.
U.S. Defense Secretary Leon Panetta said Thursday Washington was "fully prepared... to deal with any kind of provocation from the North Koreans."
Beijing, meanwhile, hinted that its patience was running low.
"If North Korea engages in further nuclear tests, China will not hesitate to reduce its assistance to North Korea," said a commentary in the Global Times, which is associated with the Communist Party.
While President-elect Park Geun-hye, who takes office Feb. 25., says the North's nuclear development in unacceptable, she says the door remains open for dialogue.
The committee said Pyongyang would "never sit down" with the South until its drops the "hostile policy."
The statement did not elaborate on what physical actions might be taken. But the North has in recent years engaged in clashes along the disputed maritime border and wage cyber attacks on Southern government bodies as media outlets.
Sejong Institute analyst Paik Hak-soon said the harsh rhetoric reflects Pyongyang's "frustration' over the new resolution, which came at a time of political transition as Japan and China also welcome new leaderships and U.S. President Barack Obama begins his second term.
"Kim Jong-un has been waiting for new leaders, who will be his partners for years to come," he said. The threats of a third nuclear test, "can be a countering element."
South Korean intelligence officials believe the North has made preparations to conduct a third nuclear test at its Punggye-ri test site, a move that would further raise tensions amid a spate of political transitions in Northeast Asia.
Many analysts believe the North will go through with the test in a bid to raise the crisis level on the peninsula.
Both Park and incumbent President Lee Myung-bak have warned that Seoul will wield a heavier stick after Pyongyang sank the warship Cheonan and shelled Yeonpyeong Island in 2010.