Korea crisis: UN's Ban warns of 'serious implications'
UN Secretary General Ban Ki-moon has appealed to North Korea to change course, saying it has "gone too far" in its rhetoric.
He said he was concerned 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 US.
However, its missiles are not believed to be capable of carrying nuclear warheads.
White House spokesman Jay Carney described the threats as "regrettable but familiar", adding the US was taking "all the necessary precautions".
Mr Ban told a news conference in Madrid 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.
Government spokesman Yoshihide Suga earlier told reporters that Japan was braced for a "worst-case scenario".
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.
Japanese and South Korean reports had suggested on Wednesday that the missile being moved by the North was a long-range one with a capability of hitting the US west coast.
However, experts believe the North's most powerful rocket, which it test-fired last December, has a range of 6,000km (3,700 miles) and can reach no further than Alaska.
South Korean Defence Minister Kim Kwan-jin played down concerns that the missile could target the US mainland, and said the North's intentions were not yet clear.
Mr Kim told MPs in a parliamentary defence committee meeting that the missile had "considerable range".
Analysts have interpreted Mr Kim's description as referring to the Musudan missile, estimated to have a range up to 4,000km. Guam would be within that range.
Declaration of war
The North is believed to have its main military research centres in the east.
It has test-fired missiles from there before, and its three nuclear weapons tests were carried out in the east.
Despite its belligerent rhetoric, North Korea has not taken direct military action since 2010, when it shelled a South Korean island and killed four people.
But in recent weeks it has threatened nuclear strikes and attacks on specific targets in the US and South Korea.
It has announced a formal declaration of war on the South, and pledged to reopen a mothballed nuclear reactor in defiance of UN Security Council resolutions.
In its latest statement, attributed to a military spokesman, the North appeared to refer to continuing military exercises between the US and South Korea in which the US has flown nuclear-capable bombers over the South.
The statement said the "ever-escalating US hostile policy towards the DPRK [North Korea] and its reckless nuclear threat will be smashed".
It promised to use "cutting-edge smaller, lighter and diversified nuclear strike means of the DPRK" and said the "merciless operation of its revolutionary armed forces in this regard has been finally examined and ratified".
The US Department of Defense said on Wednesday it would deploy the ballistic Terminal High Altitude Area Defense System (Thaad) to Guam in the coming weeks.
The Thaad system includes a truck-mounted launcher and interceptor missiles.
US officials recently also announced that the USS John McCain, a destroyer capable of intercepting missiles, had been positioned off the Korean peninsula.
Some analysts say Pyongyang's angry statements are of more concern than usual because it is unclear exactly what the North hopes to achieve.
As well as the angry statements, the North has also shut down an emergency telephone line between Seoul and Pyongyang and stopped South Koreans from working at a joint industrial complex in the North.
The Kaesong complex, one of the last remaining symbols of co-operation between the neighbours, is staffed mainly by North Koreans but funded and managed by South Korean firms.