Donald Trump has said US military solutions are "locked and loaded" against North Korea if Kim Jong-un chooses to act "unwisely".
The US president tweeted: "Military solutions are now fully in place, locked and loaded, should North Korea act unwisely. Hopefully Kim Jong-un will find another path!"
His tweet is the latest missive in an escalating war of words between Washington and Pyongyang.
Yesterday, the US president told North Korean leader Kim Jong-un he has "disrespected our country greatly", warning "with me, he's not getting away with it".
Trump's comment came after Pyongyang said it is readying a plan to fire four missiles towards the US island territory of Guam.
Trump told reporters: "He does something in Guam, it will be an event, the likes of which nobody has seen before, what will happen in North Korea."
Earlier he said North Korea could face even more than "fire and fury" if it continues to escalate tensions.
Despite the heightened rhetoric from both sides, defence secretary James Mattis said the US was committed to a diplomatic solution.
Speaking in California, Mattis said: "The tragedy of war is well enough known. It doesn't need another characterisation beyond the fact that it would be catastrophic."
- Video report by ITV News Correspondent Juliet Bremner
President Trump retweeted a US Pacific Command post that showed fighter jets and was captioned "#USAF B-1B Lancer #bombers on Guam stand ready to fulfill USFK’s #FightTonight mission if called upon to do so."
The bombers were deployed from Ellsworth Air Force Base, South Dakota to Andersen Air Force Base in Guam.
The US bombers completed a mission alongside Japan and South Korea over the East China Sea and the Korean peninsula.
The stand-off between the two countries appears to have reached its tensest point in years, with this the latest in a series of belligerent exchanges between the two leaders.
But the president did not seem intimidated by a North Korean warning that it was willing to fire missiles at Guam - a strategic US Pacific island - stating that any US response to hostilities should leave the one-party state "very, very nervous".
Most Guam residents are still relatively relaxed about the threat, said ITV News Senior International Correspondent John Irvine.
He said that it would be "an extremely provocative act" to fire missiles towards US territory - but if they are shot into the sea it might be hard for the Americans to calibrate a retaliation.
Speaking outside the White House, Trump said there were "no mixed messages" going out to North Korea.
"It [a US reaction] may be tougher than I said it, not less," the president warned.
He continued: "The people of this country should be very comfortable.
"If North Korea does anything in terms of even thinking about attack - of anybody that we love, or we represent, or our allies, or us - they can be very, very nervous.
"And they should be very nervous. Because things will happen to them like they never thought possible."
Trump also predicted the newest sanctions against North Korea, passed by the UN Security Council over the weekend, will likely be "not as effective" as intended.
"North Korea better get their act together, or they are going to be in trouble like few nations ever have been in trouble in this world."
Guam is a sovereign US territory and hosts 3,831 military personnel.
A US Air Force spokesman from Andersen Air Force base on the island said their number one priority is to be "ready to fight tonight".
He said the Guam base deters adversaries and reassures US allies and partners in the Pacific.
"We have multiple aircraft here that are part of the continuous bomber pretense."
He added: "We feel safe here right now".
Trump's comments followed a North Korean claim over state television that it was preparing to fire four missiles towards the US strategic island Guam.
The Korean Central News Agency reported that the proposal would be forwarded to leader Kim once plans had been finalised.
The threat itself came after Trump made his "fire and fury" warning, urging North Korea to abandon its nuclear ambitions.
But the president's rhetoric appeared to have little effect, with North Korean state media quoting Kim as calling Trump "bereft of reason" and talking a "load of nonsense".
Japan and South Korea have also vowed a strong reaction if the North were to go through with the plan to attack Guam.
Labour leader Jeremy Corbyn has urged the two sides to "calm down" and said Britain had a responsibility to bring the two leaders "back from the brink".
"There are phone calls that could be made, there's discussions that could be held," he said
"Surely in the interests of sanity and safety over the whole world, do it."
German Chancellor Angela Merkel said the escalating rhetoric was the "wrong answer" and saw no military solution to the crisis.