United Nations Secretary General Ban Ki-moon has appealed to North Korea to change course, saying it has "gone too far" in its rhetoric.
Mr Ban said he was concerned that any "unwanted crisis" on the Korean peninsula would have "very serious implications".
The warning came hours after South Korea's foreign minister said the North had shifted a missile with "considerable range" to its east coast.
Pyongyang earlier renewed threats of a nuclear strike against the United States. However, its missiles are not believed to be capable of carrying nuclear warheads, the BBC reports.
White House spokesman Jay Carney described the threats as "regrettable but familiar", adding that the US was taking "all the necessary precautions".
Mr Ban told a news conference in Madrid on Thursday that "nuclear threat is not a game, it's very serious ... I think they [North Korea] have gone too far in their rhetoric".
He called on all parties in the crisis to "calm down the situation and engage in dialogue".
Meanwhile, Russia said Pyongyang's attempts to "violate decisions of the UN Security Council are categorically unacceptable".
"This radically complicates, if it doesn't in practice shut off, the prospects for resuming six-party talks," foreign ministry spokesman Alexander Lukashevich said in a statement on Thursday.
The talks involving North and South Korea, the US, Russia, China and Japan were last held in late 2008.
Japan said it was co-operating closely with the US and South Korea to monitor the North's next move.
The US has responded to North Korea's statements by moving missile defence shields to Guam. The Pentagon said the shield on its Pacific island territory would be ready within weeks, adding to warships already sent to the area.
The North has previously named Guam among a list of possible targets for attack that included Hawaii and the US mainland.
