"Military solutions are now fully in place, locked and loaded, should North Korea act unwisely. Hopefully Kim Jong Un will find another path!" he tweeted.
He spoke as North Korea accused him of "driving" the Korean peninsula to the "brink of a nuclear war".
Pyongyang is finalising a plan to fire missiles near the US territory of Guam.
The Pacific island's homeland security agency issued a fact sheet on Friday with tips for residents to prepare for any missile threat.
The sheet states: "Do not look at the flash or fireball - it can blind you."
"Lie flat on the ground and cover your head. If the explosion is some distance away, it could take 30 seconds or more for the blast wave to hit."
Mr Trump's warning comes hours after US Defence Secretary James Mattis said America still hoped to solve the North Korea crisis using diplomacy.
After days of fiery rhetoric from both the US and North Korea, Mr Mattis said war would be "catastrophic" and that diplomacy was gaining results.
