James Mattis, the US defence secretary, raised the stakes over North Korea's nuclear ambitions warning that any attack on Guam could swiftly escalate into war.

Mr Mattis gave a blunt warning to Kim Jong-un as CNN reported that spy satellites had seen fresh activity at North Korean missile sites.

Last week Pyongyang announced it had plans to test-fire four missiles towards the coast of Guam, a US Pacific Island territory just over 2,000 miles away from North Korea.

Even though Pyongyang said the missiles would fall short of Guam, Donald Trump warned that Pyongyang's threats would be met by "fire and fury".