The war of words between North Korea and the U.S. is escalating. Last week, the United Nations Security Council approved new sanctions against the country, which responded with a vow of "thousand-fold revenge" against the U.S.

On Tuesday, an intelligence report that came to light concluded that North Korea has the ability to build a nuclear warhead small enough to fit on top of a ballistic missile. President Trump then warned North Korea, vowing "fire and fury like the world has never seen" if threats against the U.S. continue.

CBS News' Anthony Mason spoke with Bill Richardson, former U.S. ambassador to the United Nations and an expert on North Korea, about the increasing tensions -- and where we go from here.

What follows is a transcript of the interview, which aired August 9, 2017, on the "CBS Evening News."

ANTHONY MASON: Mr. Ambassador, where do we go after "fire and fury"? What's the best option for the U.S. now?

BILL RICHARDSON: The best option is diplomacy, the best option is continued sanctions, see if they work. Continue to pressure China, continue the military exercises, but find a way to talk to the North Koreans.

Former U.S. Ambassador to the U.N. Bill Richardson CBS News

MASON: But Kim Jong Un hasn't shown a lot of interest in diplomacy to this point.

RICHARDSON: We don't know what he wants. He's an unpredictable character. We don't know what his intentions are. He wants to stay in power. But I think in the end, once he knows that he can hit the United States with a missile, he can start negotiating. That's the way his father was. But I think keeping the talk about preemptive military strikes and the president's very incendiary statement, which was not helpful, is not the way to go.

MASON: What do you make of North Korea's threat towards Guam?

RICHARDSON: This is part of their foreign policy. However, the intensity of the attack, the specificity bothers me. The fact that the foreign minister himself -- who's a reasonable guy, I've dealt with him, Foreign Minister Re -- was so intense that I'm just a little worried that that intensity is a little too strong. What you don't want to have is a miscalculation.

MASON: What's the risk here that we could start a war by accident?

RICHARDSON: The risk is strong. A fishing boat is shot by the North Koreans, airspace is invaded and the North Koreans react, the South Koreans react -- everybody's trying to out-macho each other.

MASON: Lastly, Mr. Ambassador, U.S. intelligence seemed to be surprised by the technical advances in North Korea. Does that worry you?

RICHARDSON: That worries me because we should've been on this long ago. We should consider finding ways to put more intelligence, over-flights, more spies, because we were caught off guard. North Korea was way more advanced than our intelligence people told us. That's a massive intelligence failure that should never happen again.

MASON: Ambassador Bill Richardson, thank you so much for being with us.

RICHARDSON: Thank you.