North Korea tested two intercontinental ballistic missiles in July, claiming the US mainland is within its range. Experts say it is a matter of time before the reclusive state develops ICBMs mounted with a nuclear warhead.

Ryan said he spent a few days last week working on just the North Korea issue.

That was when tensions ran high as US President Donald Trump and the North Korean leadership traded bellicose rhetoric. Trump threatened to rain "fire and fury" on Pyongyang if it continued to threaten the US or its allies. In response, the North announced plans to fire missiles towards Guam.

The North's leader, Kim Jong-un, later backed down, saying he would watch Washington's behavior a little longer.

"This young man is an unstable person," Ryan said of Kim. "I do think he should be called to account for his reckless rhetoric and I think that's what the president's trying to do.

"The president, in my own view, he likes the unpredictability side of this," he added.

Ryan also pointed his finger at China, saying North Korea's main ally and trading partner needs to do "a lot more" to rein in the regime. (Yonhap)