Ratcheting up the rhetoric, North Korea has warned early that its military has been cleared to wage an attack on the US using "smaller, lighter and diversified nuclear" weapons.
The Pentagon, meanwhile, said in Washington that it will deploy a missile defence system to the US Pacific territory of Guam to strengthen regional protection against a possible attack from North Korea. The defence secretary said the US was seeking to defuse the situation.
Despite the rhetoric, analysts say they do not expect a nuclear attack by North Korea, which knows the move could trigger a destructive, suicidal war that no one in the region wants.
The strident warning from Pyongyang is latest in a series of escalating threats from North Korea, which has railed for weeks against joint US and South Korean military exercises taking place in South Korea and has expressed anger over tightened sanctions for a February nuclear test.
Following through on one threat, North Korean border authorities refused to allow entry to South Koreans who manage jointly run factories in the North Korean city of Kaesong.
Washington calls the military drills, which this time have incorporated fighter jets and nuclear-capable stealth bombers, routine annual exercises between the allies.
Pyongyang calls them rehearsals for a northward invasion.
The foes fought on opposite sides of the three-year Korean War, which ended in a truce in 1953. The divided Korean Peninsula remains in a technical state of war six decades later, and Washington keeps 28,500 troops in South Korea to protect its ally.
US Defence Secretary Chuck Hagel said Washington was doing all it can to defuse the situation, echoing comments a day earlier by Secretary of State John Kerry.
"Some of the actions they've taken over the last few weeks present a real and clear danger and threat to the interests, certainly of our allies, starting with South Korea and Japan and also the threats that the North Koreans have levelled directly at the United States regarding our base in Guam, threatened Hawaii, threatened the West Coast of the United States," Hagel said.
In Pyongyang, the military statement said North Korean troops had been authorised to counter US "aggression" with "powerful practical military counteractions," including nuclear weapons.
"We formally inform the White House and Pentagon that the ever-escalating US hostile policy toward the DPRK and its reckless nuclear threat will be smashed by the strong will of all the united service personnel and people and cutting-edge smaller, lighter and diversified nuclear strike means," an unnamed spokesman from the General Bureau of the Korean People's Army said in a statement carried by state media, referring to North Korea by its formal name, the Democratic People's Republic of Korea. "The US had better ponder over the prevailing grave situation."
However, North Korea's nuclear strike capabilities remain unclear.
Pyongyang is believed to be working toward building an atomic bomb small enough to mount on a long-range missile. Long-range rocket launches designed to send satellites into space in 2009 and 2012 were widely considered covert tests of missile technology, and North Korea has conducted three underground nuclear tests, most recently in February.
"I don't believe North Korea has to capacity to attack the United States with nuclear weapons mounted on missiles, and won't for many years. Its ability to target and strike South Korea is also very limited," nuclear scientist Siegfried Hecker, a senior fellow at the Center for International Security and Cooperation at Stanford University, said this week.
"And even if Pyongyang had the technical means, why would the regime want to launch a nuclear attack when it fully knows that any use of nuclear weapons would result in a devastating military response and would spell the end of the regime? " he said in answers posted to CISAC's website.
In Seoul, a senior government official said Tuesday that it wasn't clear how advanced North Korea's nuclear weapons capabilities are. But he also noted fallout from any nuclear strike on Seoul or beyond would threaten Pyongyang as well, making a strike unlikely. He spoke on condition of anonymity because he wasn't authorized to speak publicly to the media.
North Korea maintains that it needs to build nuclear weapons to defend itself against the United States. On Monday, North Korean leader Kim Jong Un led a high-level meeting of party officials who declared building the economy and "nuclear armed forces" as the nation's two top priorities.
Recent events in the escalation of nuclear tensions on the Korean Peninsula:
- March 7: The UN Security Council imposes tough sanctions against North Korea to punish it for conducting a nuclear test on February 12 in defiance of UN resolutions banning it from nuclear and missile activity. North Korea characterised the test, its third since 2006, as a defensive act against US aggression.
- March 11: South Korea and the US begin annual joint military drills. North Korea, which calls the maneuvers preparation for an invasion, responds by following through on a threat to cut a hotline with South Korea and void the 60-year-old armistice ending the Korean War.
- March 12: North Korean state media report that the country's leader, Kim Jong Un, urged front-line troops to be on "maximum alert" and warned that "war can break out right now."
- March 20: Coordinated cyberattacks in South Korea knock out computers and servers at three major TV networks and three banks. The source of the attacks remains under investigation but North Korean involvement is suspected. A week later, organisations of North Korean defectors say their computer systems also were attacked.
- March 22: North Korea condemns a UN resolution approving a formal investigation into its suspected human rights violations and says it will ignore the measure.
- March 27: North Korea cuts a military hotline to its Kaesong industrial complex, which is jointly run with the South and is the last major example of inter-Korean cooperation. Operations at the complex continue.
- March 28: In a show of force, the US takes the unprecedented step of announcing that two of its nuclear-capable B-2 bombers joined the military drills with South Korea and dropped dummy munitions on an island range. It had earlier announced the participation of older nuclear-capable B-52 bombers.
- March 29: Kim convenes an "urgent operation meeting" of senior generals just after midnight, signs a rocket preparation plan and orders his forces on standby to strike the US mainland, South Korea, Guam and Hawaii. State media quote him as saying that "the time has come to settle accounts with the US imperialists".
- March 30: North Korea warns that "inter-Korean relations have naturally entered the state of war," and says it would retaliate against any US and South Korean provocations without notice. It says hostilities "will not be limited to a local war, but develop into an all-out war, a nuclear war".
- March 31: The Central Committee of North Korea's ruling Workers' Party says the country's nuclear weapons are "the nation's life" and will not be traded even for "billions of dollars".
- April 1: The US announces it sent F-22 stealth fighter jets to participate in the US-South Korean war games.
- April 2: North Korea's atomic energy department says it will restart a plutonium reactor and a uranium enrichment plant at its main Nyongbyon nuclear complex and increase production of nuclear weapons material. The US says it would be "extremely alarming" if the North follows through.
- April 3: North Korea bars South Koreans from going to their jobs at the Kaesong industrial complex and closes the border to trucks carrying raw materials for the factories.