Sebastian Gorka said the Republican billionaire had proved he “can act” with military might after the US ‘mother of all bombs’ against ISIS and cruise missiles in Syria.
Mr Trump has warned North Korea it will be “met with fire and fury” if the rogue state continues to threaten the US.
Speaking on the Today programme on BBC Radio 4, Mr Gorka added the Republican billionaire would take action where previous presidents Barack Obama and Bill Clinton had failed to.
Presenter Justin Webb asked: “You are saying Donald Trump is ready to act?”
Mr Gorka replied the president had been firm in his response and would “use any appropriate measures”.
He said: “I’m saying Donald Trump has been unequivocal. He will use any appropriate measures to protect the United States and its citizens.”
Asked if North Korea firing methods at the sea by Guam would lead to a military attack by the US, Mr Gorka added “We don’t telegraph our future scenarios and how we’re going to react.
“That’s how the Obama administration’s thought strategy worked, that’s not how it works.
“If you show the players around the table your poker hand, you will lose that game.
“It is not a good idea in cards, it is a very bad idea in geopolitics. We will not telegraph in advance what we will or not do.
“We have sent a very clear message ‘do not challenge the United States because you will pay a cost if you do so’.
“Any other details are classified and will remain so.
“In the last six months, do we look like an administration that doesn’t act?
“I don’t think you have to worry about our decisiveness.”
Mr Gorka, a military and intelligence analyst, said the US’ position had changed since Mr Trump’s inauguration on January 20.
He added: “North Korea has said they wish to annihilate the United States and use nuclear weapons.
“Sooner or later somebody should take them seriously, the Clinton administration did not do so, the Obama administration did not do so, that stopped on January 20.
“We are not giving in to nuclear blackmail any longer.”
North Korea has threatened to fire four missiles towards the US territory of Guam as the war of words with Donald Trump edges ever-closer towards all-out conflict.
The hermit state announced will launch the strike in mid-August, with military officials preparing the final details of the attack before seeking approval from despot Kim Jong-un.
President Donald Trump pledged yesterday to respond to North Korea’s threats with “fire and fury the likes of which the world has never seen” – a move which was immediately undermined by North Korea’s declaration of war.
But Kim Jong-un’s state media said the comments were “a load of nonsense” and claimed only “absolute force can work on him”, ratcheting up tensions and leading to fears the world may be witnessing the early days of World War 3.