Speaking during a visit to South Korea Saturday, U.S. Defense Secretary James Mattis issued a stern warning the threat of North Korea launching a nuclear missile attack has definitely accelerated.

“North Korea has accelerated the threat that it poses to its neighbors and the world through its illegal and unnecessary missile and nuclear weapons programs,” Mattis warned, speaking at a press briefing in Seoul with South Korean Defense Minister Song Young-moo.

“I cannot imagine, in light of Kim’s expanded outlaw activities that the world has experienced and observed over this last year or two, I cannot imagine a condition under which the United States would accept North Korea as a nuclear power,” he said.

The Associated Press reported that Mattis visited the Demilitarized Zone on Friday, the area that acts as an official buffer between North Korea and South Korea.

“We must recognize the reality that this is a regime north of the DMZ that threatens peace and stability, and certainly directly against the people of the ROK. So our combined effort is to deter that sort of threat, or to have military options – many different military options – that would realistically reduce that threat as low as possible – and yes, we do have those options,” he promised.

“Make no mistake — any attack on the United States or our allies will be defeated, and any use of nuclear weapons by the North will be met with a massive military response that is effective and overwhelming,” Mattis stated.

Reuters noted that the CIA has estimated North Korea may be only months away from having the ability to strike the United States with nuclear weapons.

President Trump is scheduled to leave next week for a trip to Asia, where he plans to stop in South Korea to meet with President Moon Jae-in.