SEOUL, South Korea -- Angrily responding to the United Nations Security Council's unanimous decision to impose tightened sanctions, North Korea said on Friday that it was nullifying all nonaggression agreements with South Korea, with one of its top generals claiming that his country had nuclear-tipped intercontinental ballistic missiles ready to blast off.
Matching the harsh warning with a toughened stance, South Korea said Friday that if Pyongyang attacked the South with a nuclear weapon, the government of the North Korean leader Kim Jong-un would be "erased from the earth."
Such language marked the most hostile exchange between the two Koreas, still technically at war, since they engaged in an artillery skirmish three years ago.
The verbal warfare represented a clash of nerves between the young North Korean leader, who is building his credentials as head of his militaristic country, and Park Geun-hye, South Korea's first female president, who considers former Prime Minister Margaret Thatcher of Britain her role model and has stressed security as her top priority.
While weapons experts say North Korea does not have the technical capability to use nuclear-tipped missiles, that did not stop it from threatening their deployment.
"With their targets set, our intercontinental ballistic missiles and other missiles are on a standby, loaded with lighter, smaller and diversified nuclear warheads," said Kang Pyo-yong, a three-star general and vice defense minister of North Korea. "If we push the button, they will blast off and their barrage will turn Washington, the stronghold of American imperialists and the nest of evil, and its followers, into a sea of fire."
His comment, made in a speech before a large rally in Pyongyang on Thursday, was carried by the North's main party newspaper, Rodong Sinmun, on Friday.
In the last few days, North Korea's state-run news media have carried a slew of official remarks threatening to launch "pre-emptive nuclear strikes" at Washington and Seoul with "lighter and smaller nukes," hinting that the country has built nuclear warheads small enough to mount on long-range missiles. But American and South Korean officials strongly doubt that North Korea has mastered that technology, despite its successful launching of a long-range rocket in December and its third nuclear test last month.
South Korean military officials called the remarks a bluster, designed not so much to threaten Washington as to infuse its population with a sense of both crisis and empowerment as Mr. Kim consolidates his grip on North Korea. On its front page, the Rodong newspaper carried a large picture showing North Korea's new generation of mobile missiles.
South Korea's new leader warned that with its behavior, North Korea was only hurting itself.
North Korea "will collapse in self-destruction if it continue to waste its resources on nuclear weapons development while its people are going hungry," Ms. Park said during a commission ceremony for young military officers on Friday. She promised a "strong response" to any provocation, but also offered a cooperative future if North Korea changed.
Also Friday, North Korea said it was also nullifying all denuclearization agreements with South Korea and cutting off the North-South hot line, in retaliation for the United Nations sanctions and the joint military exercises South Korea is staging with the United States.
But beyond North Korea's belligerent statements, it was unclear how, if at all, Mr. Kim would react to the sanctions.
Any North Korean military action could end up involving the American forces that have remained in South Korea as it has turned from war-ravaged ruin into one of the most advanced industrialized powerhouses.
The 15-to-0 Security Council vote places potentially painful new constraints on North Korean banking, trade and travel, pressures countries to search suspect North Korean cargo and includes new enforcement language absent from previous measures. But the provisions are in some ways less important than China's participation in writing them, suggesting that the country has lost patience with the neighbor it supported in the Korean War. While China's enforcement of sanctions on North Korea remains to be seen, it may now be more assertive.
"This is not about the words, it is about the music," said Christopher R. Hill, the former American diplomat who negotiated a deal with the North during the George W. Bush administration to dismantle its nuclear facilities -- an accord that quickly collapsed. China's cosponsorship of the resolution "suggests that after many years, the screws are beginning to turn," said Mr. Hill, now the dean of the Josef Korbel School of International Studies at the University of Denver.
Still, another North Korean nuclear test is possible, as is another ballistic missile launching or perhaps an armed provocation aimed at South Korea, where Ms. Park, the daughter of a former South Korean dictator who was known for taking a hard stand with the North, could be forced to respond. Some regarded the North's dire warnings as a signal that some military response was looming.
"The higher decibel of invective is a bit worrisome," said Bill Richardson, the former governor of New Mexico and presidential candidate, who has traveled to North Korea eight times, most recently in January. "It's the highest negative level I've ever seen, and it probably means that the hard-line elements, particularly the military and not the Foreign Ministry, are in control."
On the other hand, Mr. Richardson said, "China is part of a significant sanctions effort, and this may cool the North Koreans down, may temper their response."
It is also possible that the new and isolated North Korean government may have misjudged the reaction to talk of a pre-emptive nuclear attack, wording rarely heard since the cold war ended. It could be another way in which the North is demanding talks with President Obama -- only last week Mr. Kim told Dennis Rodman, the visiting former basketball star, that he wanted Mr. Obama to call him. But it could also be a way of saying that North Korea now expected to be treated the way Pakistan is: as an established, if formally unrecognized, nuclear power.
"This is a tactic they have employed when they don't get their way, when the international community brings more sanctions to bear," said Suzanne DiMaggio, vice president of global policy programs at the Asia Society in New York. "Whether that will happen this time is unclear, given the level of hostile rhetoric," she said. "I'm not sure Pyongyang recognizes that fact." The United Nations vote and North Korea's threat come at a time when, internally, the Obama administration is debating the wisdom of its policy of essentially ignoring the North for the past four years, and responding to any provocations with new sanctions.
According to current and former administration officials, there is a growing discussion within the White House, the State Department and the Pentagon over whether Mr. Kim is using each new test of rockets and nuclear devices to solidify his position with the military, his most important single constituency. "Under that theory," one official who has dealt with North Korea often said recently, "even a firefight with the South Koreans might help him, as long as it doesn't escalate into something that threatens the regime."
In testimony on Thursday before the Senate Foreign Relations Committee, Glyn T. Davies, the administration's special representative for North Korea policy, argued that the best course was to continue with Mr. Obama's current policy of using tests and provocations to tighten sanctions, and try to starve development of the North's long-range missiles and its effort to design nuclear weapons small enough for those missiles.
Choe Sang-Hun reported from Seoul, and Rick Gladstone from New York. David E. Sanger contributed reporting from Washington.
This article originally appeared in The New York Times.