North Korea has threatened to attack South Korea if it joins a new round of tightened UN sanctions against Pyongyang, a day after the US expanded existing sanctions and denounced the communist state's decision to conduct nuclear testing.
In a third straight day of fiery rhetoric against regional powers, the North directed its verbal onslaught at its neighbour on Friday, saying: "Sanctions amount to a declaration of war against us."
"If the South Korean puppet regime of traitors directly participates in the so-called UN 'sanctions', strong physical counter-measures would be taken," the Committee for the Peaceful Reunification of Korea, said.
The warning, carried by the official Korean Central News Agency, came after North Korea's top military body on Thursday threatened to conduct a third nuclear test and boost its ability to strike the US.
Washington denounced the North for the nuclear test threat, calling the warning "needlessly provocative".
Jay Carney, White House spokesman, said such tests "would only increase Pyongyang's isolation".
"North Korea's statement is needlessly provocative and a test would be a significant violation of United Nations Security Council resolutions," he said.
Leon Panetta, the US defence secretary, said the comments were worrying.
"We are very concerned with North Korea's continuing provocative behaviour," he said at a Pentagon news conference.
The reclusive North has 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 Security Council condemned its December rocket launch.
In addition to the UN measures, Washington added further names to its own blacklist that freezes any US-based assets of designated individuals and groups and makes it a crime for anyone in the US to assist them.
Seoul has said it will look at whether there are any further sanctions that it can implement alongside the US, but said the focus for now is to follow Security Council resolutions.
A special envoy to President-elect Park Geun-hye said that South Korea's new president will not tolerate North Korean provocations but will continue to push for dialogue with Pyongyang.
The UN 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. It does not impose new sanctions on Pyongyang.
North Korea insists that its December 12 launch was a peaceful scientific mission.
The UN resolution was notable for receiving the backing of North Korea's sole major diplomatic and economic benefactor, China, which had shielded Pyongyang from stronger sanctions demanded by Washington.
In an unusually frank warning on 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 urged calm to stop the situation from deteriorating further.
"All relevant parties should refrain from action that might escalate the situation in the region," Hong Lei, foreign ministry spokesman, told reporters in Beijing.
North Korea's rhetoric this week amounted to some of the angriest outbursts against the outside world since Kim Jong-un took over the leadership of the country following the death of his father Kim Jong-il in late 2011.