James Mattis, the US defense secretary, raised the stakes over North Korea's nuclear ambitions warning that any attack on Guam could swiftly escalate into war, The Daily Telegraph reported.
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.
Speaking to Pentagon reporters Mattis, whose response to North Korea until now had been more muted, said if North Korean missiles were considered to be a threat to Guam "then it's game on".
He added: "We will try and do our best to make sure it does not hit the United States."
Mattis said the Pentagon would need only moments to ascertain where North Korean missiles were headed, adding that the US would try to shoot down anything which threatened America.
"If they fire at the United States, it could escalate into war very quickly," Mattis added.
He would not, however, be drawn on how the US would respond if it was clear that the missiles were not going to reach the island.
"If it falls short of Guam, then it becomes an issue that we take up however the president chooses," Mattis said.
"I need a certain amount of ambiguity on this because I am not going to tell them what I'll do in each case."
While Trump has talked tough over North Korea, both Mattis and Rex Tillerson, the US secretary of state, have been more cautious - emphasizing that they hoped for a diplomatic solution.
Meanwhile, US diplomats have been moving to step up economic pressure on the Kim Jong-un regime by threatening even tougher sanctions.
Earlier this month the UN Security Council unanimously backed a comprehensive series of sanctions on North Korea aimed at the country's trade in coal, iron, iron ore, lead and seafood.
Work is already underway on a fresh round of sanctions - the ninth - this time aimed at other industry's including textiles

Source: Mena