NORTH KOREA said it had "ratified" a merciless attack against the United States, potentially involving a "diversified nuclear strike".
"We formally inform the White House and Pentagon 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," a spokesman said in a statement carried by the English language service of the state news agency KCNA.
Earlier this evening, the US said it is sending an advanced ballistic missile defense system to Guam, as Chuck Hagel, the US defence secretary, warned of a "real and clear" danger from North Korea.
North Korea has threatened a nuclear strike on the United States and missile attacks on its Pacific bases, including in Guam, a U.S. territory in the Pacific, since new U.N. sanctions were imposed over the country's third nuclear weapons test in February.
One of the most isolated and unpredictable states in the world, North Korea also said on Tuesday it would revive a mothballed nuclear reactor able to produce bomb-grade plutonium.
"Some of the actions they've taken over the last few weeks, present a real and clear danger," Hagel told an audience at the National Defense University in Washington.
Hagel said he had to take the threats seriously, language he has used in recent weeks as the United States revamped its missile defense plans and positioned two guided-missile destroyers in the western Pacific to bolster missile defense.
The United States has also flexed its muscle during military drills with South Korea last week, flying two bat-winged stealth bombers on a first-of-its-kind practice bombing run over South Korea.
In the latest move, the Pentagon said on Wednesday it was deploying a Terminal High Altitude Area Defense system to Guam in the coming weeks. The THAAD system includes a truck-mounted launcher, interceptor missiles, an AN/TPY-2 tracking radar and an integrated fire control system.
"The United States remains vigilant in the face of North Korean provocations and stands ready to defend U.S. territory, our allies, and our national interests," a Pentagon spokeswoman said.
Hagel called America's responses so far "measured, responsible, serious" and also said the United States was working with allies to lower tensions.
"We are doing everything we can, working with the Chinese, others to defuse that situation on the peninsula," he said.
In Beijing, China's deputy foreign minister met ambassadors from the United States and both Koreas to express "serious concern" about the Korean peninsula, China's Foreign Ministry said. It was a sign that China, the North's major benefactor, was increasingly worried about events spinning out of control.
Despite the rhetoric, however, Pyongyang has not taken any military action and has shown no sign of preparing its 1.2 million strong armed forces for war, the White House said on Monday.
The rising tensions did not appear to jolt markets, long accustomed to cycles of rising tensions on the peninsula.
"I would say that people are taking it a lot more seriously than they used to," said Steve Van Order, a fixed income strategist at Calvert Investments in Maryland.