North Korea 'moves mid-range missile'
North Korea has shifted a missile with "considerable range" to its east coast, South Korea's defence minister says.
Kim Kwan-jin played down concerns that the missile could target the US mainland, and said the North's intentions were not yet clear.
Pyongyang earlier renewed threats of a nuclear strike against the US, though its missiles are not believed to be capable of carrying nuclear warheads.
The US is responding to North Korea by moving missile defence shields to Guam.
Meanwhile, Russia said Pyongyang's attempts to "violate decisions of the UN Security Council are categorically unacceptable".
Despite all the bluster, experts do not believe that North Korea has the capacity to launch a nuclear-armed ballistic missile at the US. However, American bases in South Korea, Japan and perhaps even as far away as Guam could well be within range of Pyongyang's conventionally armed missiles.
The US response has been a mixture of reassurance for its allies and prudent defensive precautions, including the deployment of warships with anti-missile capabilities.
Other anti-missile defences in the region are also being bolstered, a step that carries an additional message to Beijing - that if Pyongyang remains on this course anti-missile systems will only proliferate, something that may eventually compromise the effectiveness of China's own nuclear deterrent.
"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.
UN Secretary-General Ban Ki-moon appealed to North Korea to "change course" and called on all parties in the crisis to engage in dialogue.
"Nuclear threat is not a game. It's very serious," said Mr Ban. "I think they have gone too far in their rhetoric."'Worst-case scenario'
Japan said it was co-operating closely with the US and South Korea to monitor the North's next move.
Prime Minister Shinzo Abe said that although the rhetoric was "increasingly provocative", Tokyo would "calmly" watch the situation.
Government spokesman Yoshihide Suga earlier told reporters that Japan was braced for a "worst-case scenario".
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.
End Quote Sung Jin Kim, Seoul
It is unbearable to think that the lives of my loved ones could be irreversibly changed”
Japanese and South Korea reports had suggested 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.
Kim Kwan-jin told MPs in a parliamentary defence committee meeting that the missile had "considerable range".
"The missile does not seem to be aimed at the US mainland. It could be aimed at test firing or military drills," he said.
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.
US newspapers react
The New York Times says: "The Obama administration was prudent to bolster its forces in the region. Many experts assume Mr Kim won't attack the world's top military power or its allies, but Washington has an obligation to guarantee that if this assumption is wrong, it can defend the homeland."
From the Washington Post: "What the administration really needs, however, is a new strategy for answering the provocations. Diplomacy hasn't worked; neither has pressuring China to restrain the Kim regime. What has are financial sanctions targeted at the ruling elite."
The New Jersey Star-Ledger says: "While many buy into the old disarmament-for-food storyline, there's another camp concerned this episode might be different: that Kim, with little more than a year on the job, might actually believe his nation has become a nuclear power."
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".
Timeline: Korean tensions
- 12 Dec: North launches a rocket, claiming to have put a satellite into orbit
- 12 Feb: North conducts underground nuclear test
- 11 Mar: US-South Korea annual military drills begin
- 30 Mar: North says it is entering a "state of war" with South
- 2 Apr: North says it is restarting Yongbyon reactor
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.