A top South Korean security official said Sunday that North Korea may test-launch a missile this week, as the United States delayed its own missile test due to soaring tensions on the peninsula.

Kim Jang-Soo, chief national security adviser to President Park Geun-Hye, said a test-launch or other provocation could come before or after Wednesday, the date by which the North has suggested that diplomats leave Pyongyang.

North Korea, incensed by UN sanctions following its nuclear and missile tests and by South Korean-US military drills, has issued a series of apocalyptic threats of nuclear war in recent weeks.

It has also reportedly loaded two medium-range missiles on mobile launchers and hidden them in underground facilities near its east coast, raising speculation it is preparing for a provocative launch.

"There are no signs of a full-scale war as of now, but the North will have to prepare for retaliation in case of any local war," said Kim.

The spate of threats was intended to force the South and the United States to reach out with face-saving concessions, he said.

Diplomats fear the rhetoric has created a situation which could spiral out of control, and the US delayed an intercontinental ballistic missile test to avoid stoking tensions with the North.

A US defence official said Defense Secretary Chuck Hagel postponed the Minuteman 3 test at Vandenberg Air Force Base until next month due to concerns it "might be misconstrued by some as suggesting that we were intending to exacerbate the current crisis with North Korea".

And South Korea and the US postponed a major military meeting due to take place in Washington, Seoul's military said Sunday, reportedly due to fear of a provocation while Seoul's military chief is away.

Diplomats in Pyongyang huddled at the weekend to discuss a warning from the North's authorities that their safety could not be guaranteed after April 10 if a conflict broke out.

Most of their governments have made it clear they have no immediate plans to withdraw personnel, and some suggested the advisory was a ruse to fuel growing global anxiety over the crisis.

German Foreign Minister Guido Westerwelle Sunday reiterated that "any deadline after which North Korea would no longer ensure the security of embassies is unacceptable", his ministry said.

British Foreign Secretary William Hague said he saw no immediate need to withdraw his country's diplomats.

Hague also told the BBC the North is showing no sign of gearing up for "all-out conflict" by repositioning its armed forces, and called for calm.

He said there was a "danger of miscalculation by the North Korean regime which has worked itself up into this frenetic state of rhetoric in recent weeks and the danger that they would believe their own paranoid rhetoric".

"But it's also important to stress that the international response to this, including our response, must be clear, united and calm," Hague added.

China is the North's sole major ally but its patience with Pyongyang also shows signs of wearing thin.

Foreign Minister Wang Yi said China opposes "provocative words and actions" from any party in the region and would "not allow troublemaking on China's doorstep", in sharply worded comments Saturday to UN chief Ban Ki-moon.

Beijing said it had asked the North to ensure the safety of its diplomats.

But US senior Republican senator John McCain on Sunday criticised China for not doing enough to keep North Korea in check.

"China does hold the key to this problem. China can cut off their economy if they want to. Chinese behaviour has been very disappointing," he said, speaking on CBS's 'Face the Nation' talk show.

He also warned that the current situation was potentially "catastrophic" and added that "more than once, wars have started by accident".

The North's mobilised missiles are reported to be untested Musudan models which are believed to have a range of around 1,860 miles (3,000 kilometres) that could theoretically be pushed to 2,485 miles with a light payload.

That would cover any target in South Korea and Japan, and possibly even US military bases on the Pacific island of Guam.

The North has no proven inter-continental ballistic missile capability that would enable it to strike more distant US targets, and many experts say it is unlikely it can even mount a nuclear warhead on a mid-range missile.

After non-stop escalation including the public deployment of US warships and planes to the region, the Pentagon move was a welcome measure to cool tensions, said Yang Moo-Jin from Seoul's University of North Korean Studies.

"The US military may have felt that now was the time to pace itself after weeks of hectic military confrontation," he told AFP.

Facts about North Korea's Musudan missile
Seoul (AFP) April 8, 2013 - South Korean intelligence reports suggest North Korea is preparing to test-launch an intermediate-range missile at a time of escalating military tensions on the Korean peninsula.

WHAT ARE THE SIGNS A TEST COULD BE IMMINENT?

The South Korean Defence Ministry has confirmed intelligence reports that North Korea has transported at least one mid-range missile by rail to its east coast, and mounted it on a mobile launcher.

South Korean media reports that two missiles have been prepared.

WHAT KIND OF MISSILE IS IT?

The Defence Ministry has declined to identify the specific missile type, saying only that it appears to be an intermediate missile with "considerable range".

Media reports have cited senior military and government officials who identified the missile(s) as a mid-range "Musudan".

WHAT DO WE KNOW ABOUT THE MUSUDAN?

Not a great deal. The Musudan was first unveiled as an indigenous missile at a military parade in Pyongyang in October 2010, but has never been tested.

Analysis by security consultants IHS Jane's suggests the Musudan is an intermediate-range, road-mobile, liquid-propellant, single warhead missile based on the Russian R-27 and using adapted Soviet Scud technology.

Mounted on a wheeled transport-erector-launcher vehicle, it could be launched within 15 minutes of the launcher being positioned.

WHAT IS THE ESTIMATED RANGE AND PAYLOAD?

IHS Jane's puts the estimated range at anywhere between 2,500 and 4,000 kilometres (1,550 to 2,500 miles). The lower range covers the whole of South Korea and Japan, while the upper range would include US military bases on the Pacific island of Guam.

The potential payload size has been put at 1.0-1.25 tonnes.

IS IT NUCLEAR CAPABLE?

Theoretically yes, but most experts believe North Korea has yet to master the miniaturisation technology required to fit a nuclear warhead onto any of its missiles.

ARE THERE ANY DISSENTING VOICES?

Yes, German missile experts Markus Schiller and Robert Schmucker who have conducted extensive research into North Korea Korea's missile program, believe the Musudan is a "phantom missile".

The mock-up displayed in 2010 was "poorly manufactured" which suggests the North was far from a working prototype, Schmucker says.

"The Musudan, as such, does not exist," Schmucker says. "The North may well conduct a missile launch, but it will not be a Musudan. If it launches a mid-range missile, it will almost certainly be one it procured from overseas."