North Korean state media are warning of a nuclear attack on the United States at any sign of American aggression, as US warships move into the western Pacific - a force US President Donald Trump has described as an "armada".
Mr Trump, who has urged China to do more to rein in its impoverished ally and neighbour, said in a tweet that North Korea was "looking for trouble" and the US would "solve the problem" with or without Beijing's help.
Tension has escalated sharply on the Korean peninsula amid concerns that reclusive North Korea might soon conduct a sixth nuclear test, after Washington said at the weekend it was diverting an aircraft carrier strike group towards the Korean peninsula in a show of force.
"We are sending an armada. Very powerful," Mr Trump told the Fox Business Network. "We have submarines. Very powerful. Far more powerful than the aircraft carrier. That I can tell you."
Referring to North Korean leader Kim Jong-un, Mr Trump said, "He is doing the wrong thing."
Asked if he thought Mr Kim was mentally fit, Mr Trump replied: "I don't know. I don't know him."
North Korea said earlier it was prepared to respond to any US aggression.
"Our revolutionary strong army is keenly watching every move by enemy elements with our nuclear sight focused on the US invasionary bases not only in South Korea and the Pacific operation theatre but also in the US mainland," its official Rodong Sinmun newspaper said.
In spite of the military rhetoric, US officials have previously stressed that stronger sanctions are the country's most likely course to press North Korea to abandon its nuclear programme.
At the same time, Washington has said all options - including military ones - are on the table and that a US strike last week against Syria should serve as a warning to Pyongyang.
The strike group heading toward Korea includes the nuclear-powered flagship aircraft carrier USS Carl Vinson, two destroyers and a cruiser. Such a group is generally accompanied by submarines, although the Pentagon does not normally publicise this.
US Defence Secretary Jim Mattis said the deployment was not tied to a specific event but a matter of prudence.
"She is just on her way up there because that is where we thought it was most prudent to have her at this time," he told a Pentagon news conference.
White House spokesman Sean Spicer said Mr Trump had put North Korea "clearly on notice" that he would not tolerate certain actions but dismissed Pyongyang's nuclear attack threat.
"I think there is no evidence that North Korea has that capability at this time," he said. "Threatening something that you don't have the capability of isn't really a threat."
North Korea remains technically at war with the US and its ally South Korea after the 1950-1953 Korean conflict ended in a truce, not a peace treaty. It regularly threatens to destroy both countries.
However, North Korea has conducted five nuclear tests, two of them last year, and is working to develop nuclear-tipped missiles that can reach the US, presenting Mr Trump with perhaps his most pressing security headache.
Saturday is the 105th anniversary of the birth of Kim Il-sung, North Korea's founding father and grandfather of the current ruler.
A military parade is expected in Pyongyang to mark the day and North Korea often marks important anniversaries with tests of its nuclear or missile capabilities.
- Reuters / RNZ