The US is "totally prepared" to launch military action with North Korea, Donald Trump has warned.
Mr Trump said war would be "devastating" for Pyongyang - but stressed it was not the preferred option.
His comments came after North Korea accused the US of declaring "war"and threatened to shoot down American aircraft in international airspace.
On Tuesday, Mr Trump accused leader Kim Jong-un of acting "very badly", and urged the hermit nation to abandon its nuclear ambitions.
"We are totally prepared for the second option [military conflict] - not a preferred option," he said.
"But if we take that option it will be devastating for North Korea. That's called the military option."
He added: "If we have to take it, we will."
The US president argued the crisis on the Korean peninsula should have been resolved 25 years ago.
He said past administrations had failed to rein in North Korea's attempts to develop nuclear weapons, leaving him with a "mess".
Speaking at a joint press conference with Spain's prime minister, Mr Trump said: "He's [Kim] acting very badly. He's saying things that should never, ever be said."
The president continued: "North Korea is situation which should have been handled 25 years ago, 20 years ago, 15 years ago, 10 years ago, five years ago, and it could have been handled much more easily.
"Various administrations which left me a mess. But I'll fix the mess."
On Monday, the US was forced to describe as "absurd" North Korean claims the US had declared war on it.
Pyongyang's foreign minister said it interpreted the US president's latest aggressive rhetoric as a red-line threat against his country.
It came after Mr Trump threatened to "totally destroy" North Korea in an address to the UN General Assembly last week.
Ri Yong-ho said North Korea would be willing to shoot down US military jets if it felt provoked.
In a response to Mr Trump's tweets, Ri said: "The question of who won't be around much longer will be answered then."