WASHINGTON - The US Defense Secretary Jim Mattis said on Thursday that the US efforts to deal with the Korean peninsula issue was "diplomatically led," warning that war with the Democratic People's Republic of Korea (DPRK) would be "catastrophic."

"The American effort is diplomatically led, it has diplomatic traction, it is gaining diplomatic results and I want to stay right there right now," the travelling Pentagon chief told reporters in Mountain View, California.

He added that the tragedy of war was well-enough known and "it doesn't need another characterization beyond the fact that it would be catastrophic."

Mattis's remarks came amid escalating tensions between Washington and Pyongyang, casting a different tone with US President Donald Trump who unleashed a string of warnings to the DPRK.

On Tuesday, Trump issued a controversial warning to the DPRK, saying that threats from the country would be met with "fire and fury like the world has never seen."

Earlier on Thursday, Trump escalated his threat, claiming that maybe his previous warning "wasn't tough enough."

In fact, Mattis himself had also uttered strong words against the DPRK and reminded Pyongyang of the US military strength before his latest remarks.

"The DPRK must choose to stop isolating itself and stand down its pursuit of nuclear weapons," Mattis said in a written statement on Wednesday, adding that the DPRK's "actions will continue to be grossly overmatched by ours and would lose any arms race or conflict it initiates."