WASHINGTON -- US House Speaker Paul Ryan said Monday that he worries most about North Korea selling a nuclear-tipped long-range missile to terrorists.

The Republican congressman from Wisconsin told an audience at CNN's Town Hall that that is the reason the North Korea issue is serious.

"Having North Korea with nuclear-tipped warheads that can strike the United States is not a good thing to have for this country," he said. "But it's not just that they would launch a missile to the United States. What I worry most about is that they'll sell one. I worry that they're going to proliferate, that they're going to sell it to the highest bidder, to a terrorist."

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)