"The American people should be concerned about a state that has developed a pretty good ICBM capability, and is developing a pretty good nuclear reentry vehicle," he told reporters at the White House, referring to intercontinental ballistic missiles.

"Right now, we think the threat is manageable, but over time, if it grows beyond where it is today, well, let's hope diplomacy works," he said.

Tensions have run high as North Korea has launched a series of ballistic missiles, including two ICBMs, and conducted its sixth and most powerful nuclear test in recent months.

A war of words between Trump and the North Korean leadership has further fueled the tensions.

"I think I speak for the administration that that state simply cannot have the ability to reach the homeland," Kelly said. (Yonhap)