China has issued a veiled criticism of North Korea by saying no country should be allowed to plunge a region into chaos, after the United States postponed a missile test to ease talk of war.

The North, led by 30-year-old Kim Jong-un, has been issuing threats of war against the United States and US-backed South Korea since the United Nations imposed sanctions after its third nuclear weapon test in February.

Its anger has been compounded by weeks of joint military exercises by South Korea and the US.

The North told diplomats late last week to consider leaving Pyongyang because of the tension, but embassies appeared to view the appeal as more rhetoric and staff have stayed put.

South Korea said it was anticipating any manner of action the North's unpredictable leaders might make, including a possible missile launch, by Wednesday, after when the North said it could not guarantee diplomats' safety.

China, North Korea's sole financial and diplomatic backer, has shown growing irritation with North Korea's warnings of nuclear war.

Chinese President Xi Jinping, addressing a forum on the southern island of Hainan, did not name North Korea yesterday but said no country "should be allowed to throw a region and even the whole world into chaos for selfish gain".

Stability in Asia, he said, "faces new challenges, as hot-spot issues keep emerging and both traditional and non-traditional security threats exist".

Chinese Foreign Minister Wang Yi expressed similar frustration, saying: "We oppose provocative words and actions from any party in the region and do not allow trouble-making on China's doorstep."

Yesterday, the ministry expressed "grave concern" at rising tension and said China had asked North Korea to "ensure the safety of Chinese diplomats in North Korea, in accordance with the Vienna Convention and international laws and norms".

UK Foreign Minister William Hague said North Korea's nuclear ambitions had to be taken seriously. Interviewed by Sky News, he said the international response "should also be very clear, very united and calm at all times because it's important not to feed that frenetic rhetoric that we've seen over the last few weeks".

Switzerland's Foreign Ministry offered to mediate, saying it was "always willing to help find a solution, if this is the wish of the parties, such as hosting meetings between them".

Mr Kim, the third member of his dynasty to rule North Korea, is thought to have spent several years in Switzerland being educated under a pseudonym. He took power in December 2011 after the death of his father, Kim Jong-il, who confronted South Korea and the US throughout his 17-year rule.

In Washington, a defence official said a long-scheduled test of the Minuteman III intercontinental missile, due to take place at the Vandenberg Air Force Base in California, would be postponed.

He said the test had been unconnected to "anything related to North Korea" and added another test launch could be expected next month. The US remained fully prepared to respond to any North Korean threat, the official said.

The South Korean president's office said the country had a "firm military readiness" for any eventuality. "As of now, there are no signs of all-out war, but if a local conflict breaks out, North Korea should be aware it will pay the price," the chief of the National Security office was quoted as saying.