North Korea has threatened to attack South Korea if Seoul joins a new round of tightened UN sanctions, as Washington unveils more of its own economic restrictions following Pyongyang's rocket launch last month.
In a third day of fiery rhetoric, North Korea directed its verbal onslaught at its neighbour on Friday, saying: "'Sanctions' mean a war and a declaration of war against us".
The reclusive country this week declared a boycott of all dialogue aimed at ending its nuclear programme and vowed to conduct more rocket and nuclear tests after the UN security council censured it for a long-range missile launch in December and expanded existing sanctions.
"If the puppet group of traitors takes a direct part in the UN 'sanctions', the DPRK [Democratic People's Republic of Korea] will take strong physical counter-measures against it," North Korea's committee for the peaceful reunification of Korea said, referring to its neighbour.
The committee is Pyongyang's front for dealings with Seoul.
On Thursday, the US placed economic sanctions on two North Korean bank officials and a Hong Kong trading company that it accused of supporting Pyongyang's proliferation of weapons of mass destruction.
The company, Leader (Hong Kong) International Trading, was blacklisted by the United Nations on Wednesday.
Seoul has said it will look at whether there are any further sanctions it can implement alongside the US, but said the focus for now is to follow security council resolutions.
The resolution said the council "deplores the violations" by North Korea of its previous resolutions, which banned Pyongyang from conducting further ballistic missile and nuclear tests and from importing materials and technology for those programmes. It does not impose new sanctions on Pyongyang.
The US had wanted to punish North Korea for the rocket launch with a security council resolution that imposed new sanctions against Pyongyang, but China rejected that option. Beijing agreed to UN sanctions against Pyongyang after North Korea's 2006 and 2009 nuclear tests.
North Korea's rhetoric this week amounted to some of the angriest outbursts against the outside world since Kim Jong-un took over as leader after the death of his father, Kim Jong-il, in late 2011.
On Thursday, North Korea said it would carry out further rocket launches and a nuclear test, directing its ire at the US, a country it called its "sworn enemy".
The US defence secretary, Leon Panetta, said the comments were worrying.
"We are very concerned with North Korea's continuing provocative behaviour," he said at a Pentagon news conference. "We are fully prepared … to deal with any kind of provocation from the North Koreans. But I hope in the end that they determine that it is better to make a choice to become part of the international family."
North Korea is not believed to have the technology to deliver a nuclear warhead capable of hitting the continental US, although its December launch showed it had the capacity to deliver a rocket that could travel 6,200 miles (10,000km), potentially putting San Francisco in range, according to an intelligence assessment by South Korea.
Seoul and others who have been closely observing activities at North Korea's known nuclear test grounds believe Pyongyang is technically ready to go ahead with its third atomic trial and is awaiting the political decision of its leader.
North Korea's committee also declared on Friday that a landmark agreement it signed with South Korea in 1992 on eliminating nuclear weapons from the peninsula was invalid, repeating its long-standing accusation that Seoul was colluding with Washington.
The foreign ministry of China, North Korea's sole remaining major diplomatic and economic benefactor, repeated its call for calm on the peninsula at its daily briefing on Friday.
"The current situation on the Korea peninsula is complicated and sensitive," spokesman Hong Lei said. "We hope all relevant parties can see the big picture, maintain calm and restraint, further maintain contact and dialogue, and improve relations, while not taking actions to further complicate and escalate the situation."
But unusually prickly comments in Chinese state media on Friday hinted at Beijing's exasperation.
"It seems that North Korea does not appreciate China's efforts," said the Global Times in an editorial, a sister publication of the official People's Daily. "Just let North Korea be 'angry' … China hopes for a stable peninsula, but it's not the end of the world if there's trouble there. This should be the baseline of China's position."
