The US has delayed an intercontinental ballistic missile test to avoid stoking tensions with North Korea, as fears escalated that weeks of angry rhetoric could erupt into conflict on the Korean peninsula.

The Pentagon's disclosure that it would reschedule the test due in California next week comes as the international community grows increasingly nervous that the situation could spiral out of control.

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".

"We wanted to avoid that misperception or manipulation," the US official told AFP. "We are committed to testing our ICBMs to ensure a safe, secure, effective nuclear arsenal."

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 intermediate-range missiles on mobile launchers and hidden them in underground facilities near its east coast, raising speculation it is preparing for a provocative launch.

Foreign 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.

British Foreign Secretary William Hague said Sunday 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.

The top national security adviser to South Korea's President Park Geun-Hye said Sunday the warning was another ploy to force the South and the United States to reach out with face-saving concessions.

"We believe the North is trying to turn the situation around by making the US send a special envoy, the South to offer dialogue and China or Russia to act as a mediator," Kim Jang-Soo said.

China is the North's sole major ally but its patience with Pyongyang 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.

South Korea's Kim, a former defence minister, warned a missile launch by the North was possible around the April 10 date given to foreign embassies, but said there was no sign it is preparing for a ruinous full-scale conflict.

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 reach 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.

"If the North really launches intermediate-range missiles as widely feared, the US may be partially blamed for having pushed it to take such drastic action by deploying extremely threatening weaponry near the Korean peninsula."

Western tourists returning from organised tours in Pyongyang -- which have continued despite the tensions -- said the situation there appeared calm.

"We're glad to be back but we didn't feel frightened when we were there," said Tina Krabbe, from Denmark.

North Korea on Wednesday put in place a ban on South Koreans accessing their companies in the Seoul-funded Kaesong industrial zone on the North side of the border. There are no cross-trips on Sundays.

The estate is the only surviving example of inter-Korean cooperation and seen as a bellwether for stability.

But Seoul said Sunday that 13 South Korean firms there had so far been forced to suspend production because of a shortage of materials or personnel.