US plays down North Korea threat

North Korea has asked foreign countries to consider evacuating their embassies in Pyongyang, amid soaring tension on the Korean peninsula following a month-long war of words between the North and the South.

Warnings by the North that nuclear war could break out at any time has prompted the US to strengthen its Pacific missile defences and Russia to express its concerns.

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.

Britain is one of several European countries including Russia to receive a letter regarding the safety of its embassy in Pyongyang, as North Korea moved two mid-range missiles to its east coast.

The Foreign Office said on Friday that North Korea had warned Britain that it could not protect foreign embassies after April 10 in the event of a conflict.

"The DPRK has responsibilities under the Vienna Convention to protect diplomatic missions and we believe they have taken this step as part of their country's rhetoric that the US poses a threat to them," the Foreign Office spokesperson said.

It said it had "no immediate plans" to evacuate its embassy and accused the North Korean government of raising tensions "through a series of public statements and other provocations"

Earlier on Friday, South Korea's Yonhap news agency reported that North Korea had moved two medium-range missiles to its east coast.

The report cited a senior military official as saying: "Early this week, the North has moved two Musudan missiles on the train and placed them on mobile launchers."

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".