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In a statement issued in the name of its Committee for the Peaceful Reunification of the Fatherland, which manages relations with South Korea, North Korea gave no hint on what those countermeasures might be. While its earlier pronouncements more often than not turned out to be a bluster, North Korea does have a history of following up some with unexpected military attacks — most recently, its shelling of a border island in 2010 that left four South Koreans dead. It was also blamed for the sinking of a South Korean warship the same year that left 46 sailors dead.

Those two incidents brought the two Koreas closer to waging a full-scale war than ever in recent decades, dispelling Washington's desire to engage North Korea for a serious negotiation. In the last few days, while calling for a vigorous enforcement of U.N. sanctions, U.S. officials also appealed to the North's new leader, Kim Jong-un, not to overreact and miss the opportunities for a new beginning.

Pyongyang's threat against South Korea was the latest in a verbal barrage it has launched since the United Nations Security Council on Tuesday unanimously adopted a resolution condemning North Korea's Dec. 12 rocket launching as a violation of earlier U.N. resolutions banning it from testing ballistic missile technology. The resolution called for tightening sanctions to cut off the procurement activities for North Korea's nuclear and missile programs, while the North accused Washington of masterminding the Security Council to "stifle" the already impoverished country.

"If the puppet group of traitors takes a direct part in the U.N. 'sanctions,' the D.P.R.K. will take strong physical countermeasures against it," North Korea said on Friday, using the nickname it often uses for the South Korean government and the acronym of its official name, the Democratic People's Republic of Korea. "'Sanctions' mean a war and a declaration of war against us."

The U.N. resolution was the fifth to be slapped on the North for its rocket and nuclear programs since 1993. It calls for the tightening of existing sanctions, such as expanding a travel ban on North Korean officials and the freezing of assets of North Korean banks and other agencies accused of engaging in shipments and financing for the North's missile and nuclear programs. It also broadened the means for U.N. member nations to intercept and confiscate cargo headed for the North.

Since the Security Council resolution, North Korea has said it would conduct a nuclear test and launch more long-range rockets and that there would be no more talks on the "denuclearization" of the Korean Peninsula, a main goal of Washington's thus-far unsuccessful diplomacy on the Korean Peninsula for the last two decades.

With Friday's threat against the South, North Korea, under the young Mr. Kim, appeared to be following a well-worn track established under his late father, Kim Jong-il, before his death in December 2011: a cycle of North Korean provocation such as a rocket launching, U.N. condemnation, North Korean warnings of "physical countermeasures," which were sometimes followed by provocative actions, such a nuclear test.

While this familiar cycle repeated itself in recent years, North Korea also steadily boosted its nuclear and missile capabilities. The North Korean nuclear crisis began in the early 1990s with nothing but a tiny amount of fissile material North Korea was suspected of gleaning from its experimental research reactor. It has since accumulated enough plutonium for an estimated half dozen nuclear bombs, built a full-scale uranium-enrichment program, conducted two nuclear tests and made strides toward building intercontinental ballistic missiles that U.S. officials feared could one day be tipped with nuclear warheads.

On Thursday, North Korea said it felt no need to hide its intention of building rockets and nuclear weapons with the United States as a "target" because Washington had intensified its "hostile" policy against the North.

On Friday, North Korea directed its ire at its neighbor, South Korea, warning that Seoul should expect a continuing confrontation and even potential military clashes on the Korean Peninsula if the hard-line policy of the outgoing President Lee Myung-bak was inherited by his successor, President-elect Park Geun-hye, who will be sworn in next month.

"Now that the South Korean puppet conservative group is more desperately kicking up a racket against the D.P.R.K. over its nuclear and missile issues with the U.S., there will be no more discussion on denuclearization between the north and the south in the future," North Korea said. "As long as the South Korean puppet group of traitors persistently pursues a hostile policy toward the D.P.R.K., we will never negotiate with anyone."

In its statement, the North also said that a 1992 joint declaration in which the two Koreas committed themselves not to purse nuclear weapons was now completely invalid. It was not the first time North Korea has called its deals with Seoul and Washington nullified. Still, analysts said, the North's posture significantly limits room for Ms. Park's overtures toward the North; like Mr. Lee and President Obama in the United States, Mr. Park considers the dismantling of the North's nuclear program the premise in all South Korea's diplomacy toward the North.

The U.N. sanctions and the North's angry reactions dissipated early hopes that changes of leadership in Pyongyang, Seoul and Washington might open the way for easing tensions. But some analysts said that North Korea was just escalating tensions ahead of dialogue to increase its leverage.

North Korea, which has lived through U.S.-led trade embargoes throughout its existence, considers itself a small yet proud nation struggling for independence in the face of an "imperialist" plot to erase its from the earth. It has typically called any new round of American-inspired sanctions a declaration of war.

Washington says North Korea is one of the leading threats to global efforts for nuclear and missile non-proliferation.