The official said such a move would be a “game-changer” if it actually happened but raised questions about Pyongyang’s technical capabilities.
He said: “There’s a certain amount of bluster that’s taken for granted when you’re dealing with North Korea.”
North Korea said it might test a hydrogen bomb over the Pacific after Donald Trump vowed to destroy the country, with leader Kim Jong Un promising to make the US President pay dearly for his threats.
Kim did not specify what action he would take against the US or Mr Trump but described him as a “mentally deranged US dotard”.
Mr Trump ramped up the rhetoric by re-issuing his threat in a message on Twitter and dismissing Kim as a “madman”.
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He said: “Kim Jong-un of North Korea, who is obviously a madman who doesn’t mind starving or killing his people, will be tested like never before.”
Earlier in the week, Mr Trump dubbed Kim “rocket man on a suicide mission” and told the United Nations General Assembly he would “totally destroy” North Korea if it threatened the US and its allies.
Kim said he would consider the “highest level of hard-line countermeasure in history” against the US and that Mr Trump’s comments only confirmed his own nuclear program was “the correct path”.
Pyongyang conducted its sixth and largest nuclear test on September 3 and has launched dozens of missiles this year as it accelerates a program aimed at enabling it to target the US with a nuclear-tipped missile.
US Secretary of State Rex Tillerson said diplomatic efforts will continue but all military options were still on the table.
He said: “We are quite challenged with the escalating rhetoric but hope the increased sanctions and voices from every corner of the world will help lead Kim to talks.”
China, Russia and South Korea have all called for calm.
Chinese foreign ministry spokesman Lu Kang said: “All relevant sides should exercise restraint and dedicate themselves to easing the situation rather than irritating each other.”