Kim warned that his rocket forces were ready "to settle accounts with the U.S." Friday, thousands of soldiers in North Korea marched in support of their leader. According to state media, Kim "convened an urgent operation meeting" of senior generals just after midnight, signed a rocket preparation plan and ordered his forces on standby to strike the U.S. mainland, South Korea, Guam and Hawaii.
Kim said "the time has come to settle accounts with the U.S. imperialists in view of the prevailing situation," according to a report by the North's official Korean Central News Agency.
North Korean state media on Saturday said the country is in a "state of war" with South Korea, and has intentions to attack America. The statement by Pyongyang said all matters between the sides will be dealt with in a manner befitting war.
"We have to take seriously every provocative, bellicose action that this new young leader has taken so far," said Defense Secretary Chuck Hagel.
The North Koreans have been issuing a number of threats toward South Korea and the U.S. in recent weeks. In response, the Air Force carried out drills with South Korea that included the flight of two B-2 stealth bombers.
The bombers flew nonstop from their base in Missouri to South Korea, where they dropped dummy munitions at a military range.
The Pentagon said this was the first time a B-2 had dropped dummy munitions over South Korea, and later added that it was unclear whether there had ever been any B-2 flights there at all.
The mission follows a string of threats from the North. In the most dramatic case, Pyongyang made the highly improbable vow to nuke the United States. Experts believe the country is years away from developing nuclear-tipped missiles that could strike the United States. Many say they've also seen no evidence that Pyongyang has long-range missiles that can hit the U.S. mainland.
Still, the North's threats are worrisome because of its arsenal of short- and mid-range missiles that can hit targets in South Korea and Japan. Seoul is only a short drive from the heavily armed border separating the Koreas.
Some experts believe the threats from the North are part of an effort to bolster the image of Kim in his home country. U.S. officials say they will not be intimidated.
"If there was a war, North Korea would lose, they would lose badly. The Kim regime would end as we know it. North Korea would end as we know it. North Korea does not want that to happen," said Jim Walsh.
The U.S. launched the B-2 mission just hours after North Korea cut off communications with the South. Most analysts think the rise in rhetoric from the North is in part an effort to force concessions and aid from the south and the U.S.
"We will unequivocally defend and we're unequivocally committed to that alliance with South Korea," said U.S. Defense Secretary Chuck Hagel. "We will be prepared if we have to be prepared to deal with any eventuality there."
Experts say the biggest concern is that with tensions so high, the chance of a military engagement breaking out accidentally is also high. Military leaders determined nearly 100,000 U.S. troops would be needed to storm the country and secure North Korea's nuclear material.
The Associated Press contributed to this report.