"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, issuing his own sharp threat to the country.

"The DPRK should cease any consideration of actions that would lead to the end of its regime and the destruction of its people."

He also asserted that any action by North Korea would be grossly overmatched by the U.S., and that Pyongyang would lose any arms race or conflict it started.

Mattis issued the statement as he traveled to the West Coast. His comments punctuated President Donald Trump's warning that North Korea will be met with "fire and fury" if it threatens the U.S. His threats also come amid U.S. efforts for diplomatic solutions, the combined military power of America and its allies is the most robust on Earth.

A new report said that Pyongyang's nuclear program is progressing.