US plays down North Korea threat
US officials have played down the threat of war on the Korean peninsula, after weeks of bellicose statements from Pyongyang.
A White House spokesman said the US "would not be surprised" if North Korea launched a missile, while a top US military officer said recent threats appeared to fit a familiar pattern.
Pyongyang has threatened to attack both US and South Korean targets.
It has told foreign embassies it cannot guarantee their safety in a conflict.
Diplomats in Pyongyang were asked on Friday to tell the foreign ministry by 10 April what help they would need in evacuating.
The warning prompted Russia to ask whether Pyongyang was offering help in the event of a conflict, or making a decision.
South Korean media reported on Friday that the North had moved two intermediate range missiles into position on the east coast.
The missiles are untested but it is believed they could reach as far as the Pacific Island of Guam, where the US has a military base, and where it has confirmed it will deploy a missile defence system.
Yonhap, the South Korean news agency, said two warships equipped with Aegis defence systems would monitor the situation.
North Korea has issued a series of unusually strong threats since it was sanctioned by the UN in March for having carried out a third nuclear test.
It has threatened nuclear strikes on the US, formally declared war on the South, and pledged to reopen a nuclear reactor in defiance of UN Security Council resolutions.
White House spokesman Jay Carney said on Friday a missile launch would not be unexpected.
"We would not be surprised to see them take such an action,'' he said. "We have seen them launch missiles in the past.''
Seoul has also played down the North's reported missile move, saying it may be planning a test rather than a hostile act.
Gen Martin Dempsey, chairman of the US Joint Chiefs of Staff, called the North's nuclear threat "reckless", but said it seemed to fit a decades-long pattern of escalation followed by accommodation.
"I wouldn't say I see anything to lead me to believe that this is a different kind of cycle,'' he told the Associated Press news agency.
Even so, Gen Dempsey said the cycle was more unpredictable because relatively little was known about North Korean leader Kim Jong-un, who came to power after his father's death in December 2011.
"Though we've always said that North Korea has been a bit opaque to us, in the past we've understood their leadership and the influencers a little better than we do today," he said.
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 B2 and B52 bombers over the South as part of the drill, and has since deployed warships with missile defence systems to the region.
Gen Dempsey said US moves had been "largely defensive and exclusively intended to reassure our allies".
The BBC's Lucy Williamson reports from Seoul that the heightened atmosphere could make any action riskier for North Korea.
With military communication lines cut, even an unarmed test-flight of its rockets could be misconstrued, and any glitch in flight path or target could lead to a major escalation, she adds.
North Korea has not taken direct military action since 2010, when it shelled a South Korean island and killed four people.
Despite its warning that it could not guarantee the safety of foreign embassies, both Russia and the UK said they had no immediate plans to evacuate their embassies in Pyongyang.