Secretary Mattis attacked North Korea in a statement as he stood feet away from Kim Jong-un’s prison state.
He said: “North Korean provocations continue to threaten regional and global security despite unanimous condemnation by the United Nations Security Council.”
He added that the hermit kingdom was developing nuclear weapons to “threaten others with catastrophe”.
The DMZ visit by General Mattis is expected to be seen as the latest escalation in tension between the US and North Korea.
In a robust statement, the General backed South Korea against further aggression and promised to defend the ally from its neighbour.
He vowed to stand “shoulder to shoulder” with Seoul in the face of threats from despot Kim.
The world is still waiting to see whether President Trump will visit the DMZ when he arrives in South Korea.
When pressed on the on Wednesday he said: “Well, I’d rather not say, but you’ll be surprised.”
North Korea yesterday labelled Donald Trump a “lunatic” and warned that his visit to the South will mean war in a chilling threat to the President.
The Hermit Kingdom has long threatened violence against the President and the US, once claiming they would wipe the America “off the face of the earth”.
Now a statement from state propaganda mouthpiece KCNA has warned Trump could “bring a war to the Korean peninsula” by visiting South Korea next month.
The KCNA missive also branded the US President a “war maniac” and a “lunatic”.
Trump could enter North Korea in a direct provocation to Kim Jong-un if he visits the DMZ during his trip to Seoul next month.