UN Assistant Secretary of State, Susan Thornton, said: “If a missile targeted Guam or flew over it, we would have shot it down,” in a briefing for foreign journalists.
Ms Thornton added that in Japan there are special alert systems for the civilian population.
She said: ”The question of whether or not to apply military actions depends on specific situation.”
President Donald Trump threatened to “destroy” North Korea if Pyongyang continues its nuclear programme, during his maiden speech at the United Nations.
The remarks were met with fury from Kim Jong-un who made a rare broadcast appearance, where he branded the US President as a “dotard” and “frightened dog”.
The US leader responded with a tweet calling Kim Jong-un “a madman” who “will be tested like never before!”
Dr Emily Landau, a senior research fellow at the Institute for National Security Studies (INSS), told the Express.co.uk that she finally thinks the dictator is realising there will be serious consequences – potentially nuclear ones – for continually threatening the United States.
Kim has previously threatened to fire missiles towards the US Pacific territory of Guam and claimed the hermit kingdom had developed weapons which could reach the US mainland.
US President Donald Trump rapidly responded saying threats like that would be met with “fire and fury”.
Dr Landau went onto add: “I think Kim might fire towards Guam and then we’ll have to see what the response will be.
She said Guam is a “grey area” for the US and she does not believe it is covered by the fire and fury threat, despite being a US territory.