"The DPRK (Democratic People's Republic of Korea) must choose to stop isolating itself and stand down its pursuit of nuclear weapons," Mattis said in a statement.
"The DPRK should cease any consideration of actions that would lead to the end of its regime and the destruction of its people."
He added that: "The DPRK regime's actions will continue to be grossly overmatched by ours and would lose any arms race or conflict it initiates.