North Korea is preparing for a fourth nuclear test, according to a South Korean minister, who said Seoul had been alerted to increased activity at the country's main atomic test site.

When asked about reports the North was stepping up activity at the underground site Punggye-ri, where they carried out a third nuclear test in February, the South Korean Unification Minister Ryoo Kihl-jae said" "there are such signs". He failed to elaborate any further.

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The Punggye-ri test site in January

It was possible for the North to carry out a missile strike and conduct a nuclear test at the same time according to Kim Min-seok, a spokesperson for South Korea's defence ministry.

However he played down the threat to South Korea's official news agency Yonhap saying: "Following the North's nuclear test in February, we had briefed that not only the western tunnel but also the southern tunnel were made ready for another test. As there are several facilities near the nuclear test site, vehicles and people could come and go. Currently, there is no new movement to add on to the previous briefing."

United Nations Secretary-General Ban Ki-moon said carrying out a fourth nuclear test would be a "provocative" act that would breach Security Council resolutions.

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There has been increasingly bellicose rhetoric from the North

On Sunday, William Hague called for calm over tensions in the Korean peninsula but added Britain "should be concerned" about North Korea's activities.

The Foreign Secretary said there was a "danger of miscalculation" over Kim Jong Un's regime, which had worked itself up into a "frenetic state of rhetoric."

He stressed there were no signs that North Korea was beginning to re-position its forces ready for war.

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William Hague has appealed for calm

Speaking on BBC 1's Andrew Marr Show on Sunday, he said:"It is important to stress that we haven't seen in recent days, in recent weeks, a change in what is happening in North Korean society. We have not been able to observe that. We've haven't seen the repositioning of forces or the redeployment of ground forces that one might see in a period prior to a military assault or to an all out conflict."

However he added: "There's a threat to the world from any country breaching the non proliferation treaty, which North Korea is doing, acting in contravention of a whole series of UN Security Council resolutions and setting out to develop more and more longer range weapons, testing new nuclear weapons and indulging in the proliferation of many items to other countries as well.

"We should be concerned about that. There is a danger in that.

Hague declined to speculate on whether the 29-year-old dictator was "nuts".

He said: "I don't know the man myself of course. It's not easy, although we have an embassy there, it's not easy to get face to face discussions with North Korean leaders so I'm not going to speculate about the psychology of the leader of North Korea except in the terms that I have already spoken about - that authoritarian and totalitarian regimes perfectly rationally from the point of view of their own survival in the short term often do this sort of thing, try to ramp up an external threat."

It comes after North Korea announced on Friday that it will not guarantee the safety of international embassies in Pyongyang if war breaks out.

The Foreign Office has said it has "no immediate plans to withdraw'' Britain's embassy in Pyongyang and condemned the "provocations'" of the North Korean regime.

The United States announced on Sunday it had cancelled a planned test of a Minuteman 3 intercontinental ballistic missile (ICBM) over fears of escalating tensions on the Korean peninsula.

The test, which was due to take place next week from the Vandenberg Air Force Base in California, has been delayed for a month following a series of increasingly bellicose threats from Pyongyang.