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Intelligence chiefs fear they could launch cyber attacks on nuclear power stations and air traffic control centres in the hope of plunging the country into chaos.

MI5 and MI6 believe they entered using fake passports and posing as South Korean students.

They are thought to be part of a North Korean secret unit called the General Bureau of Reconnaissance, which conducts clandestine operations against “enemies of North Korea”.

(Image: PA/REUTERS) (Image: GETTY - STOCK IMAGE)

They would have been acting on the orders of Kim Jong-un the country’s supreme leader.

The group, who are trained in assassinations, unarmed combat and sabotage, are part of an elite band of North Koreans who are allowed to leave the country.

It is believed that members of the unit were involved in the assassination of Kim’s half-brother Kim Jong-nam at Kuala Lumpur International Airport this year.

There is also a concern that they could try to poison water supplies with nerve agents or lethal toxins.

A senior defence source said: “The agents are dangerous because they have been trained for most their lives to obey orders without question and without fear.”

The Daily Star Sunday has also learned that the US has over 150 military satellites monitoring activity within North Korea.

(Image: REUTERS)

A range of military options from a nuclear strike to targeted attacks have also been drawn up by US defence chiefs.

But sources have also revealed that both the UK and the US have put pressure on China to resolve the problem by convincing Kim that he needs to back down or will face a destructive war.

A British Government source said: “It’s difficult to imagine China allowing Kim to start a war.”

In the meantime, Britain will send a Trident submarine to the region, while RAF AWACS aircraft could assist the US in the event of a pre-emptive strike against the regime.