HARI SREENIVASAN: We return to the North Korea story.

Moments ago, the regime's news agency said its leader, Kim Jong-un, has been briefed on plans for a missile test that would splash down near the U.S. territory of Guam.

And, as we reported earlier, Defense Secretary Mattis vowed today to shoot down any missile fired.

But where the regime is getting the engines for its new missiles is the subject of provocative new report. Its author says a factory in Ukraine is the source.

We go to special correspondent Nick Schifrin for more.

NICK SCHIFRIN: Joining me now is Michael Elleman. He's a senior fellow for missile defense at the International Institute for Strategic Studies and a former consultant to the Pentagon. And Melissa Hanham is a senior research associate at the James Martin Center for Nonproliferation Studies at the Middlebury Institute.

And thank you both for being here.

Mike, let's start with you, because you wrote the paper.

You come out with a notion that North Korea has advanced from middle-range missiles to ICBMs fast than any country has. And your notion is they got technology, they stole the technology, specifically from Eastern Ukraine. What's the evidence?

MICHAEL ELLEMAN, International Institute for Strategic Studies: The engines they are using for the longer-range missiles has an appearance that's very similar to a well-known engine family that originates in Russia and Ukraine.

And I have talked with sources that are — that have been to some facilities in Ukraine in the recent time, and they have seen the modifications that would have to be made from this existing engine of Russian-Ukrainian heritage to the one we see in the North Korean missiles.

And that happened rather recently, and one even bragged about having made the transformation or the remodeling of the pumping system. And then you look at the performance characteristics of the engine in the North Korean missiles, you get almost a precise match of capability.

And when you combine all those, it leads me to believe that sources in either Ukraine or Russia allowed these engines to make their way to North Korea.

NICK SCHIFRIN: Now, Melissa, I want to turn to you.

This paper is not without its critics. And you and I have discussed whether the paper actually underestimates whether North Korea itself can have built this engine, rather than have stolen it.

MELISSA HANHAM, Middlebury Institute of International Studies at Monterey: It's more likely that they are perhaps borrowing some design influence, but not actually importing engines at all.

I think it's really common to believe that North Korea is backwards, that, you know, one of the common memes we see is that its lights are off at night. Really, what's happened in North Korea is that they have marshaled their resources, limited though they are, towards their military program.

And the new engines that we are seeing now have marked differences from others, and, in many cases, I think it is safe to say that they are largely indigenous, though that there are commonalities between all engines.

NICK SCHIFRIN: And, Mike, what's wrong with that? Is North Korea not capable of building these kinds of engine, or perhaps there are Ukrainian-Russian workers inside of North Korea?

MICHAEL ELLEMAN: Well, we haven't seen an indigenously developed engine before this.

Most countries start with smaller engines and slowly build up to much larger ones. The one we're talking about here that's powering their long-range missiles generates the equivalent of 40 tons of thrust, which is an enormous amount of energy.

One would expect them to start off with smaller engines in an indigenous design. So, when I look at those things, it's just hard for me to believe that suddenly the first engine North Korea has ever made produces this much thrust and it's successful, by the way, in each one of these tests.

NICK SCHIFRIN: Mike, I have talked to members of the intelligence community today. They actually echo Melissa's argument that North Korea has more capacity than you give them credit for.

And the Ukrainian government, as you know, has come out against your report today. Alexander Turchinov, he's the secretary of National Security and Defense Council. He said: "Ukraine has never supplied rocket engines and any missile technology to North Korea. We believe that this anti-Ukrainian campaign" — that's a reference to you — "was triggered by Russian secret services to cover their participation in the North Korean nuclear and missile programs."

Is Ukraine wrong, or it possible the Ukrainian government doesn't know?

MICHAEL ELLEMAN: Well, I would argue that they don't know.

In my article, I specifically say that it's very unlikely that the government itself was involved. I would be very surprised if even executives from the Yuzhmash factory were involved.

I think this is more likely to be rogue actors. There are a number of elicit arms traders throughout the former Soviet Union, including Russia, including Ukraine. Let's not forget that the location of the Yuzhmash factories is only a couple hundred kilometers from where there is an active battle going between Ukraine and the separatists in Eastern Ukraine.

And this often gives rise to a lot of illicit markets. So, I'm not surprised the Ukrainian government is upset about the reporting, but I'm just trying to report on the facts as I know them. And I can assure you I'm not working for the Russian government.

NICK SCHIFRIN: Melissa, wrap this up for us quickly.

Is there any fear, from your perspective, that the longer we assume the North Koreans are backwards or not capable, the quicker we run out of time?

MELISSA HANHAM: The Chamjin missile factory is famous for production of missile parts. And their procurement activities over the last few years have definitely shown that they have imported the machine tools that they need in order to produce these types of engines and other parts of missiles that are needed.

I don't know where you would get a Nodong engine if they were not produced locally in North Korea, for example. Many of the photographs of Kim Jong-un visiting the facilities are underground. And I think that's why there isn't a lot of talk about them, but they do exist.

And I think, as you mentioned earlier, the longer we wait in order to engage North Korea in negotiation, the more likely their technical capabilities and the sheer numbers of these missiles and trucks will go up.

NICK SCHIFRIN: Melissa Hanham, Mike Elleman, thank you very much.

MICHAEL ELLEMAN: Thank you.

MELISSA HANHAM: Thank you.