The North Korean Army has warned Washington that its military has been cleared to launch a "merciless attack," possibly involving nuclear weapons.
The threat early on April 4 is the latest in a series of threats from Pyongyang directed at the United States and South Korea.
An unnamed army spokesman said in a statement carried by the Korean Central News Agency that troops have been authorized to counter U.S. aggression with "powerful, practical military counteractions."
"We formally inform the White House and Pentagon," the statement said, "that the ever-escalating U.S. hostile policy toward the DPRK (North Korea) 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 of the DPRK and that the merciless operation of its revolutionary armed forces in this regard has been finally examined and ratified."
The warning came after the Pentagon said it will deploy a missile-defense system to the U.S. Pacific territory of Guam to beef up its defense capabilities in the region.
On April 4, South Korea's defense minister said North Korea had moved a missile with "considerable range" to its east coast. But Kim Kwan-jin dismissed reports it was a missile capable of reaching the United States.
South Korean Defense Ministry spokesman Wi Yong-sub said the country's military was prepared to deal with the threats.
"Our weapons are ready," he said. "Regarding North Korea's statement today, our military readiness is already high. We're able to manage any possible crisis."
Japanese government spokesman Yoshihide Suga said that Pyongyang's inflammatory rhetoric would not intimidate Tokyo.
"Our government does not believe we should be pushed around and swayed by every provocative statement out of North Korea," he said. "We are coordinating with the relevant nations to enact the various [UN] Security Council resolutions. We also strongly urge North Korea to understand that these provocative statements are not in the interest of their nation. "
U.S. Defense Secretary Chuck Hagel said on April 3 that Washington takes North Korea's threats seriously.
"They have nuclear capacity now. They have missile delivery capacity now, so as they have ratcheted up their bellicose, dangerous rhetoric," Hagel said. "And some of the actions they have taken over the last few weeks present a real and clear danger and the threat to the interests, certainly of our allies, starting with South Korea and Japan, and also the threats that the North Koreans have leveled directly at the United States."
Analysts say North Korea lacks the means to strike the U.S. mainland but could target U.S. troops in South Korea or Japan.
Meanwhile, North Korean authorities on April 4 barred, for the second day in a row, South Koreans from coming to work in a jointly run industrial area just over the border.
The Kaesong complex is one of the impoverished regime's only sources of hard currency, earning it an estimated $80 million a year.
North Korea on April 4 repeated its threat to shut down the Kaesong industrial zone if the South's government continued what Pyongyang called its "confrontational ways."
Tensions have been rising since Pyongyang's third nuclear test in February led to a tightening of international sanctions against the regime.
With reporting by Reuters and AP