Mr Trump tweeted that “military solutions are now fully in place”, adding: “Hopefully Kim Jong Un will find another path!”
The US President’s warning came hours after China appealed to the US and North Korea “to be cautious with their words and actions” amid heightened nuclear tensions.
On Thursday, Mr Trump met with national security advisers to discuss Pyongyang’s threat to fire missiles towards the US territory of Guam.
In an address after the meeting, the President suggested his warning to hit North Korea with “fire and fury” may not have been tough enough.
In a subsequent attempt to ease tensions, defence secretary James Mattis told reporters that the US still prefers a diplomatic approach to easing tensions as a war would be “catastrophic”.
However, when asked whether America was prepared to respond if North Korea commits a hostile act, he replied: “We are ready.”
Later, Pyongyang accused Washington of a “criminal attempt to impose nuclear disaster upon the Korean nation” and described the US as a “heinous nuclear war fanatic”.
Mr Trump’s “locked and loaded” threat was met with a mixed reaction among world leaders.
German Chancellor Angela Merkel said the President’s escalation of rhetoric was “the wrong answer”, adding: “I do not see a military solution and I don’t think it’s called for”.
Meanwhile, Russian foreign minister Sergei Lavrov said the threats from Washington and Pyongyang were “going over the top”.
He added that “the side that is stronger and cleverer” should take the first step to defuse the crisis.
The latest war of words between the US and North Korea started after the latter test-fired two missiles in July.
Despite the tensions, American and South Korean officials say they still plan to stage joint military drills from 21 August.
Washington and Seoul say the annual exercises, which involve tens of thousands of troops, are defensive in nature and crucial to maintaining a deterrent against North Korean aggression.