North Korea threatened the South on Friday and said sanctions passed this week were a "declaration of war," even as China and the United States sought to pressure Pyongyang into backing down on a planned nuclear test.
In the latest in a series of bellicose warnings from Pyongyang sparked by a tightening of United Nations sanctions, the North's top body for inter-Korean affairs threatened the South with unspecified "physical countermeasures."
"Sanctions amount to a declaration of war against us," the Committee for Peaceful Reunification of Fatherland said in a statement.
"If the South Korean puppet regime of traitors directly participates in the so-called UN 'sanctions,' strong physical countermeasures would be taken," it added.
The warning, carried by the official Korean Central News Agency, came a day after North Korea's top military body threatened to conduct a third nuclear test and boost its ability to strike the United States.
The current upsurge in tensions has its roots in Pyongyang's defiant decision to push ahead with a long-range rocket launch on Dec. 12, insisting it was a peaceful mission to place a satellite in orbit.
The rest of the world saw it as a banned ballistic-missile test and, on Tuesday, the UN Security Council unanimously adopted a resolution that expanded the number of North Korean entities on an international blacklist.
The United States, supported by Japan and South Korea, spearheaded the UN resolution.
Pyongyang reacted furiously, vowing to boost its nuclear arsenal and to conduct a third nuclear test and even longer-range rocket launches in an "all-out action" against its "sworn U.S. enemy."
The UN resolution was notable for receiving the backing of North Korea's sole major ally, China, which had shielded Pyongyang from stronger sanctions demanded by Washington.
After meetings late Friday with Chinese officials in Beijing, U.S. envoy Glyn Davies said the two sides had "achieved a very strong degree of consensus," saying recent comments from Pyongyang were "troubling and counterproductive."
Mr. Davies, the U.S. special representative for North Korea policy, also said they "talked about the implementations of the UN Security Council resolution and the Chinese assured us that they would, of course, follow and implement that resolution, and we take them at their word."
Outgoing U.S. Defence Secretary Leon Panetta has said the United States was "fully prepared" for a test from Pyongyang.
"But I hope … they determine that, in the end, it is better to become a part of the international family," Mr. Panetta said.
In an unusually frank warning earlier Friday, China's state-run media indicated that Beijing would decrease aid to Pyongyang if it goes ahead with a nuclear test.
"If North Korea engages in further nuclear tests, China will not hesitate to reduce its assistance," the Global Times, which is close to China's ruling Communist Party, said in an editorial.
"China hopes for a stable peninsula, but it's not the end of the world if there's trouble there," it added.
As North Korea's main economic lifeline, China is seen as the only country with any genuine leverage over the impoverished, isolated and nuclear-armed state.
Pyongyang has long played on Chinese fears of the consequences of North Korea's collapse to defy Beijing's efforts to rein in its nuclear-weapons program.
Mushrooming trade with China has helped North Korea mitigate the impact of UN sanctions already imposed after Pyongyang's previous nuclear tests in 2006 and 2009.
The Global Times editorial expressed official displeasure at a lack of North Korean understanding of Beijing's role in softening the sanctions outlined in Tuesday's UN resolution.
"It seems that North Korea does not appreciate China's effort," the newspaper said.