The BBC's Lucy Williamson in Seoul: "North Korea is unpredictable and the stakes are very high”
North Korea has told foreign embassies in Pyongyang it cannot guarantee their safety in the event of conflict, and to consider evacuating their employees.
The UK Foreign Office said it was "considering next steps". Russia said it had no plans to evacuate.
The North's move comes amid threats to attack US and South Korean targets.
South Korea has reportedly deployed two warships with missile-defence systems after the North was said to moved a missile to its east coast.
Military officials told South Korean media the two warships would be deployed on the east and west coasts.
Seoul has played down the North's missile move.
It said the move may be for a test rather than a hostile act.
Creating a crisis?British diplomats said on Friday the North had asked them to respond by 10 April on what support the embassy would need in the event of any evacuation - and they were considering their moves.
Russian diplomats said they had no immediate plan to evacuate, stressing there were no outward signs of tension in Pyongyang.
But anecdotal reports from inside the capital, Pyongyang, say the mood there is calm, and many believe North Korea is deliberately trying to create a sense of crisis, says the BBC's Lucy Williamson in Seoul.
One of the US targets named by Pyongyang was the Pacific island of Guam, which hosts a US military base.
On Thursday, the US confirmed it would deploy a missile-defence system to Guam in response to the threats.
"The moves that we have been making are designed to ensure and to reassure the American people and our allies that we can defend the United States," said state department spokeswoman Victoria Nuland.
Footage has been released of North Korean President Kim Jong-un watching a military drill
South Korea's foreign minister told MPs on Thursday that the North had moved a missile to the east coast, which is the location for previous military tests.
Unconfirmed reports on Friday said the North had moved two missiles, and had loaded them onto launchers.
Aegis missile defence system
- Allows warships to shoot down enemy ballistic missiles while they are still in space
- Interceptors are fired to hit missiles before they re-enter the atmosphere
- The US, South Korea and Japan all have Aegis capability
The missiles are thought to be mid-range Musudans, which are untested in flight but are thought to have the capacity to reach as far as Guam.
The South Korean news agency Yonhap said that two warships equipped with Aegis defence systems would monitor the situation.
"If the North fires off a missile, we will trace its trajectory," Yonhap quoted an official as saying.
'Turn down volume'Despite North Korea's belligerent rhetoric, it 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 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.
Regional papers reflect on crisis
In South Korea
- Chosun Ilbo says: "The military has pledged to maintain a solid defence amid increasing threats from North Korea, but incident after incident shows how empty that pledge is."
- JoongAng Daily writes: "The escalation of tension by the North has hardly affected the South. Pyongyang's provocations are aimed at consolidating Kim Jong-un's power base at home."
In Japan
- According to Yomiuri Shimbun: "Kim should be keenly aware that the pursuits of nuclear armament and economic reconstruction are incompatible."
- Asahi Shimbun writes: "The United States and its two regional allies, Japan and South Korea, should start working closely together in serious efforts to figure out the best way to deal with North Korea."
Many of North Korea's angry statements have cited the annual military exercises between US and South Korean forces as provocation.
The US flew nuclear-capable B-2 and B-52 bombers over the South as part of the drill, and has since deployed warships with missile-defence systems to the region.
North Korea's official media say the US is surrounding the peninsula with a nuclear threat from land, sea and air.
Reports in US media quoting unnamed Pentagon officials suggest Washington is now questioning whether some of its actions may have contributed to the tension.
CNN quoted a Pentagon official as saying the US would now try to "turn the volume down" on its rhetoric.
In recent weeks, the North has shut down an emergency military hotline 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.
