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A North Korean military spokesman announcing the end of the armistice that ended the Korean War in 1953.
South Korea's military said it will strike back at North Korea and target its top leadership if Pyongyang launches a threatened attack.
A top North Korea general, in a rare appearance on state television on Tuesday, threatened military action against the U.S. and South Korea because of military drills between the two western allies countries that began March 1.
Tensions have ratcheted higher across the Korean peninsula since the North, under youthful leader Kim Jong Un who took office just over a year ago after the death of his father, launched a long-range rocket last December.
The youngest son of Kim Jong Il succeeded his late father in 2011, becoming the third member of his family to rule the unpredictable and reclusive communist state.
He followed this with a third nuclear test on February 12, triggering the prospect of more U.N. sanctions that are due to be formally announced on Thursday after the United States and China, the North's one major diplomatic ally, struck a deal to punish Pyongyang.
At the same time, North Korea has stepped up its military threats against South Korea and the United States, prompting the terse warning from Seoul on Wednesday that it would not stand idly by if its territory was attacked.
"We have all preparations in place for strong and decisive punishment, not only against the source of the aggression and its support forces but also the commanding element," Major General Kim Yong-hyun of the South Korean army told a press conference.
North Korea's bellicose rhetoric rarely goes beyond that, although in 2010 it sank a South Korean naval vessel, killing 46 sailors and in the same year shelled a South Korean island, killing civilians.
South Korea's new President Park Geun-hye had pledged to engage with the North if it dropped its nuclear plans but now faces the prospect of a hostile challenge early in her 5-year term.
The proposed fresh sanctions would explicitly ban the sale to Pyongyang of items coveted by North Korea's ruling elite, such as yachts and racing cars, a U.N. Security Council diplomat said on condition of anonymity.
In 2009, Italian authorities blocked the sale of two yachts worth more than $10 million that they believed were headed for Kim Jong Il, the current Kim's father, who enjoyed copious amounts of luxury brandy and fresh sushi in a country where a third of the population is malnourished.
The new sanctions will target North Korea's financial transactions, which often involve using cash couriers that make them hard to trace, and its criminal activities such as drugs and counterfeiting.
North Korea continues military drills and exercises in support of a top general's threat to back military action against South Korea and the United States. NBCNews.com's Dara Brown reports.
North Korea was slapped with sanctions in 2006 that banned the import of a range of luxury goods from jet skis to Harleys following its first nuclear test in a bid to hit the high-life of the Kim family and its hangers-on.
The impoverished country, whose economy is smaller than it was 20 years ago, has been subject to sanctions of some kind from the United States for almost all of its existence and since 2006 has seen U.N. sanctions imposed for its long range rocket and nuclear tests.
Despite the sanctions Pyongyang now has a nuclear stockpile sufficient for around half a dozen warheads, has made substantial progress in developing a long-range missile and is working towards miniaturizing a nuclear warhead for an intercontinental ballistic missile.
China has backed all rounds of sanctions and fell into line with the latest move in the Security Council, risking relations with its prickly ally.
About 200,000 Korean troops and 10,000 U.S. forces are expected to be mobilized for their "Foal Eagle" exercise, under the Combined Forces Command, which goes until the end of April. Separate computer-simulated drills called "Key Resolve" start on March 11.