UN Secretary General Ban Ki-moon told North Korea on Thursday to rein in its nuclear threats, warning that any miscalculation could lead to an extremely grave outcome.
North Korea has issued a crescendo of nuclear threats over past weeks, culminating in a military statement Thursday that it had received approval for action possibly including nuclear strikes.
The daily security and humanitarian reports from Pyongyang are "really alarming and troubling", the United Nations chief told a news conference in Madrid.
"Nuclear threat is not a game, it is very serious," Ban told reporters after meeting with Spanish Prime Minister Mariano Rajoy.
"I think they have gone too far in their rhetoric and I am concerned that if by any misjudgement, by any miscalculations ... this will have very serious implications," the UN boss said.
Ban, a former South Korean foreign minister, said he had been repeatedly appealing to Pyongyang to reduce tensions and engage "proactively and constructively" for peace and security on the Korean peninsula.
He called on all parties to work together to ease tensions and open dialogue.
South Korea said the North had moved a medium-range missile to its east coast as the United States strengthened its Pacific missile defences amid the intensifying threats from Pyongyang.
Seoul's defence minister Kim Kwan-Jin said the missile could reach a "considerable distance" but not the US mainland.
"It could be aimed at test-firing or military drills," he told lawmakers Thursday.
Earlier in the day, North Korea's general staff, angered by UN sanctions and South Korea-US military drills, issued a statement saying: "The moment of explosion is approaching fast".
It said the US use of nuclear-capable B-52 and B-2 stealth bombers in war games with South Korea was provocative.
The US aggression would be "smashed by... cutting-edge smaller, lighter and diversified nuclear strike means," it said.
The Pentagon said it would send ground-based THAAD missile-interceptor batteries to protect bases on Guam, a US territory some 3,380 kilometres (2,100 miles) southeast of North Korea and home to 6,000 American military personnel.