North Korea has issued another warning, a day after announcing plans for a third nuclear test.
In a statement, Pyongyang pledged "physical counter-measures" against South Korea if it participated in the UN sanctions regime.
The threat came 24 hours after North Korea said it would proceed with a "high-level" nuclear test in a move aimed at "arch-enemy" the US.
The White House condemned the move, labelling it "needlessly provocative".
North Korea has conducted two nuclear tests in the past, in 2006 and 2009. It gave no time-frame for its third test.
Its announcement followed the adoption by the UN Security Council of a resolution condemning North Korea's recent rocket launch and extending sanctions.
North Korea says its rocket launch was for the sole purpose of putting a satellite into orbit; the US and North Korea's neighbours say it was a test of long-range missile technology banned under UN resolutions.
'Declaration of war'
The second warning in two days came in a statement from the Committee for the Peaceful Reunification of the Fatherland, carried by KCNA news agency.
"If the puppet group of traitors takes a direct part in the UN 'sanctions', the DPRK (North Korea) will take strong physical counter-measures against it," it said, referring to the South Korean leadership.
"'Sanctions' mean a war and a declaration of war against us."
The UN resolution, passed on Tuesday, expanded existing sanctions against Pyongyang that were imposed after its previous nuclear tests and rocket launches.
Washington has also expanded its own sanctions against North Korea, with targets including a Hong Kong-based trading company and two North Korean bank officials based in Beijing.
On Thursday, it spoke out against a third nuclear test.
"Further provocations would only increase Pyongyang's isolation, and its continued focus on its nuclear and missile programme is doing nothing to help the North Korean people," White House spokesman Jay Carney told reporters.
Beijing has called for dialogue, urging all parties to act with restraint and "look at the long-term interest".
But an editorial in China's state-run Global Times appeared to hint at exasperation.
"If North Korea engages in further nuclear tests, China will not hesitate to reduce its assistance," the editorial said.
Both North Korea's previous nuclear tests followed long-range rocket launches.