SEOUL - North Korea threatened to attack rival South Korea if Seoul joined a new round of tightened UN sanctions, as Washington unveiled more of its own economic restrictions following Pyongyang's rocket launch last month.

In a third straight day of fiery rhetoric against regional powers, the North directed its verbal onslaught at its neighbor on Friday, saying: "'Sanctions' mean a war and a declaration of war against us."

The reclusive North has this week declared a boycott of all dialogue aimed at ending its nuclear program and vowed to conduct more rocket and nuclear tests after the Security Council censured it for a December long-range missile launch.

"If the puppet group of traitors takes a direct part in the UN 'sanctions,' the DPRK will take strong physical counter-measures against it," the North's Committee for the Peaceful Reunification of Korea said, referring to the South.

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