Enraged over the South's current joint military drills with the United States and last week's U.N. sanctions imposed on Pyongyang for its Feb. 12 nuclear test, North Korea has piled threat on top of threat, including a vow to launch a nuclear strike on the U.S.
Seoul has responded with tough talk of its own and has placed its troops on high alert. Tensions on the divided peninsula have reached their highest level since North Korea rained artillery shells on a South Korean island in 2010.
The North Korean government made no formal announcement on its repeated threats to scrap the 60-year-old armistice, but the country's main newspaper, Rodong Sinmun, reported that the armistice was nullified Monday as Pyongyang had said it would.
The North has threatened to nullify the armistice several times before, and in 1996 it sent hundreds of armed troops into a border village. The troops later withdrew.
Despite the North Korean report, U.N. spokesman Martin Nesirky said the armistice is still valid and still in force because the armistice agreement had been adopted by the U.N. General Assembly and neither North Korea nor South Korea could dissolve it unilaterally.
"The terms of the armistice agreement do not allow either side unilaterally to free themselves from it," said Nesirky, the spokesman for U.N. Secretary-General Ban Ki-moon.
Ban urged North Korea "to continue to respect the terms of the armistice agreement as it was approved by the General Assembly," Nesirky said, adding that officials at U.N. headquarters in New York were unaware of any operational changes on the ground on the Korean peninsula.
White House spokesman Jay Carney said the U.S. was "certainly concerned by North Korea's bellicose rhetoric. And the threats that they have been making follow a pattern designed to raise tension and intimidate others."
He added that Pyongyang "will achieve nothing by threats or provocation, which will only further isolate North Korea and undermine international efforts to ensure peace and stability in northeast Asia."
U.S. National Security adviser Tom Donilon told the Asia Society in New York that Pyongyang's claims may be "hyperbolic," but the United States will protect its allies.
"There should be no doubt: we will draw upon the full range of our capabilities to protect against, and to respond to, the threat posed to us and to our allies by North Korea," Donilon said in remarks prepared for delivery. "This includes not only any North Korean use of weapons of mass destruction but also, as the president made clear, their transfer of nuclear weapons or nuclear materials to other states or non-state entities. Such actions would be considered a grave threat to the United States and our allies and we will hold North Korea fully accountable for the consequences."
Despite the heightened tensions, there were signs of business as usual Monday.
The two Koreas continue to have at least two working channels of communication between their militaries and aviation authorities. One of those hotlines was used Monday to give hundreds of South Koreans approval to enter North Korea to go to work. Their jobs are at the only remaining operational symbol of joint cooperation, the Kaesong industrial complex. It is operated in North Korea with South Korean money and know-how and a mostly North Korean workforce.
The 11-day military drills that started Monday involve 10,000 South Korean and about 3,000 U.S. troops. Those coincide with two months of separate U.S.-South Korean field exercises that began March 1.
The drills are held annually, and this year, according to South Korean media, the "Key Resolve" drill rehearses different scenarios for a possible conflict on the Korean peninsula using computer-simulated exercises. The U.S. and South Korean troops will be used to test the scenarios.
Also continuing are large-scale North Korean drills that Seoul says involve the army, navy and air force. The South Korean Defense Ministry said there have been no military activities it considers suspicious.
The North Korean rhetoric escalated as the U.N. Security Council on Thursday approved a new round of sanctions over Pyongyang's nuclear test.
Analysts said that much of the bellicosity is meant to shore up loyalty among citizens and the military for North Korea's young leader, Kim Jong Un.
"This is part of their brinksmanship," said Daniel Pinkston, a Seoul-based expert on North Korea with the International Crisis Group think tank. "It's an effort to signal their resolve, to show they are willing to take greater risks, with the expectation that everyone else caves in and gives them what they want."
Part of what North Korea wants is a formal peace treaty to end the Korean War, instead of the armistice that leaves the peninsula still technically in a state of war. It also wants security guarantees and other concessions, direct talks with Washington, recognition as a nuclear weapons state, and the removal of 28,500 U.S. troops stationed in South Korea.
Pinkston said there is little chance of fighting breaking out while war games are being conducted, but he added that he expects North Korea to follow through with a somewhat mysterious promise to respond at a time and place of its own choosing.
North Korea was responsible for an artillery attack that killed four South Koreans in 2010. A South Korean-led international investigation found that North Korea torpedoed a South Korean warship that same year, killing 46 sailors. Pyongyang denies sinking the ship.
Among other recent threats, North Korea has warned Seoul of a nuclear war on the divided peninsula and said it was canceling nonaggression pacts.
South Korean and U.S. officials have been closely monitoring Pyongyang's actions and parsing its recent rhetoric, which has been more warlike than usual.
One analyst said Kaesong's continued operations show that North Korea's cutting of the Red Cross communication channel was symbolic. More than 840 South Koreans were set to cross the border Monday to Kaesong, which provides a badly needed flow of hard currency to a country where many face food shortages, according to Seoul's Unification Ministry.
"If South Koreans don't go to work at Kaesong, North Korea will suffer" financially, said analyst Hong Hyun-ik at the private Sejong Institute in South Korea. "If North Korea really intends to start a war with South Korea, it could have taken South Koreans at Kaesong hostage."
Under new President Park Geun-hye, South Korea's Defense Ministry, which often brushes off North Korean threats, has looked to send a message of strength in response to the latest comments from Pyongyang.
The ministry has warned that the North's government would "evaporate from the face of the Earth" if it ever used a nuclear weapon. The White House also said the U.S. is fully capable of defending itself against a North Korean ballistic attack.
On Monday, Park told a Cabinet Council meeting that South Korea should strongly respond to any provocation by North Korea. But she also said Seoul should move ahead with her campaign promise to build trust with the North.
North Korea has said the U.S. mainland is within the range of its long-range missiles, and an army general told a Pyongyang rally last week that the military is ready to fire a long-range nuclear-armed missile to turn Washington into a "sea of fire."
While outside scientists are still trying to determine specifics, the North's rocket test in December and third nuclear test last month may have pushed the country a step closer to acquiring the ability to hit the U.S. with weapons of mass destruction. Analysts, however, say Pyongyang is still years away from acquiring the smaller, lighter nuclear warheads needed for a credible nuclear missile program.
But there are still worries about a smaller conflict, and analysts have said that more missile and nuclear tests are possible reactions from North Korea.
North Korea has a variety of missiles and other weapons capable of striking South Korea. Both the warship sinking and island shelling in 2010 occurred near a western sea boundary that North Korea fiercely disputes. It has been a recurring flashpoint between the rivals that has seen three other bloody naval skirmishes since 1999.
Last week, Kim Jong Un visited two islands just north of the sea boundary and ordered troops there to open fire immediately if a single enemy shell is fired on North Korean waters.
Kim was also quoted as saying his military is fully ready to fight an "all-out war" and that he will order a "just, great advance for national unification" if the enemy makes even a slight provocation, according to the North's official Korean Central News Agency.
Associated Press writers Peter James Spielmann at the United Nations, and Nedra Pickler, Matthew Pennington and Bradley Klapper in Washington contributed to this report.