How should the world respond to North Korea's bombshell claim?

GWEN IFILL: We return now to North Korea's bombshell claims today.

Siegfried Hecker is former director of Los Alamos National Laboratory, and he now teaches at Stanford University. He has visited North Korea and its nuclear facilities seven times. And Wendy Sherman was undersecretary of state for political affairs during the Obama administration. During the Clinton administration, she was a State Department official focused on North Korea. And she now advises the Hillary Clinton campaign.

Wendy Sherman, how much of a threat is this latest test?

WENDY SHERMAN, Former Obama administration official: Well, I think the fact that North Korea has now conducted a fourth nuclear test is of concern.

I agree with the White House's assessment, at least so far — and I will be glad to hear what Mr. Hecker has to say — that this isn't a hydrogen test, but, nonetheless, four nuclear tests is a concern to us.

GWEN IFILL: So, the fact of the test is more concerning to you than the type of test?

WENDY SHERMAN: Well, no. Had it been a hydrogen bomb — and, of course, it will still be several days before we know that for sure — that is much, much, much more powerful.

And, of course, we're also concerned about miniaturization of the weapon, so that it might be carried on a weapon to South Korea, or Japan or even to the United States. So, I this is an acceleration and escalation by North Korea. And I think that the United States and the world community need to act with resolve.

GWEN IFILL: Siegfried Hecker, tell us, what exactly is possible for North Korea to have pulled off in this case?

SIEGFRIED HECKER, Former Director, Los Alamos National Laboratory: Well, it's not clear exactly what they did.

As Ms. Sherman pointed out, they did another nuclear test. And, actually, my greatest concern is that they did the fourth nuclear test. With that nuclear test, they clearly achieved greater sophistication, in most likelihood meaning being able to make the bomb smaller, lighter, and, therefore, have greater reach, if they're able to get them on a missile.

Whether it was a hydrogen test or not, we don't know. My sense is, we will probably never really know. So far, from what is the information that's there, it appears the size of the blast, in other words, the power from the seismic signal that can be measured, it's about the same level as the third nuclear test in 2013, which we put at the level of approximately a Hiroshima bomb. That's saying 10 to 15 kilotons.

Now, whether it actually achieved the sophistication of going to a fusion bomb — that is, the normal bombs are fission or atomic bombs — the fusion or hydrogen bomb, that is a very big step technologically. And it's not clear that that was done, but we also can't rule out that the North Koreans have made a significant advancement.

GWEN IFILL: Wendy Sherman, we talked about the U.S. reaction. Let's walk through some of the other intersectional reaction, especially China.

In the past, they have been as protective of North Korea as any nation. Not so today.

WENDY SHERMAN: Right.

Well, today, they said they are resolutely opposed to what North Korea has done. Now we need some resolute action from China as well. It is significant and useful that the U.N. Security Council held an emergency session. The reports out of that session are that nobody objected to further sanctions. And Ambassador Power, the U.S. ambassador, called for more sanctions.

And many candidates today who are running for president have called for greater action. And Secretary Clinton, who certainly understands what's happening in Asia, has called for greater sanctions and to ensure, as I think we all feel, that we not allow North Korea to blackmail the international community, but that we take resolute action to tell them, this is not acceptable.

GWEN IFILL: Siegfried Hecker, let's talk about resolute action for a moment. A year ago, the president signed an executive order to freeze assets in North Korea.

Everyone, as Wendy Sherman pointed out, is calling for resolute action, universal condemnation. What difference does that make for a rogue state like North Korea?

SIEGFRIED HECKER: Quite frankly, I think none, because we have been through this at least since 2003 or so, when North Korea pulled out of a nonproliferation treaty.

And the attempts, not only by the United States, but by the international community, has been, in essence, to threaten North Korea, to sanction North Korea, to isolate North Korea. And it simply hasn't worked. I think we failed to engage North Korea appropriately when we had opportunities in these last 12 or 13 years.

Whatever engagement was there didn't work. The bottom line is, over this time, from 2003, when they most likely built their first primitive device which they tested in 2006, until today, they have gone from building a device, 2003, testing one that didn't work so well in 2006, to just now where they had the fourth test, the successful test.

And, in the meantime, at the same time, they have scaled up their ability to make more bombs. And so where we used to have a problem of having this country that could perhaps build a simple nuclear device, today, they appear to have a nuclear arsenal. That's of great concern.

And, to me, that means we have to do something different than what's been done over the last 12 years.

GWEN IFILL: Wendy Sherman, we know that Kim Jong-un is a tough read. Not many people have gotten inside and figured out what he's really up to. But if from — based on what we know, if what he was trying to do today was provoke, what was that provocation intended to do?

WENDY SHERMAN: Well, I think he's — it's several things.

First of all, it's really to bring together his own country to believe that he is strong and powerful. He is still a young leader. He's trying to consolidate his power. He's done that through a tyrannical set of actions, including killing off some of his closest advisers, when he thought they were getting out of control and he wasn't being seen as primary.

So this was a way to bring his country together, which we find completely reprehensible, but, nonetheless, he's taken that action. Second, he's sending a message to the region and to the United States that he will do whatever he thinks he needs to do to protect his country. He will not go down, as other leaders have gone down around the world.

I take Sieg's statements, and I understand them, the frustration here to get North Korea to do something, but to get action takes a couple of things. It takes a leader who is willing to engage. And North Korea has not been willing to engage, though the United States and others have made many entreaties to them to do so.

And, secondly, I think we have to get China, who has really one of the only relationships left with North Korea, along perhaps a little bit with Russia, to not only engage, but also to take tougher action to take away some of the goodies that they still provide to North Korea.

GWEN IFILL: And, finally, Siegfried Hecker, your take on Kim Jong-un and whether he is a real threat in this situation.

SIEGFRIED HECKER: So, I like to be able to say that I am just a scientist, and so these diplomatic matters are certainly beyond my own personal reach.

But the thing that seems clear, just in terms of the nuclear weapons piece of this, is that North Korea looks at these nuclear weapons as a deterrent. During all of my visits and the discussions with their Foreign Ministry and the diplomats, rather than the technical people, they talked about their deterrent.

So the emphasis was always deterrent, meaning deterring the United States from essentially, you know, going into North Korea, or, as they like to say, you know, our hostile policies.

So, I think we need to understand exactly what is the North Korean security concern, because without getting over that concern, in North Korea, to come to resolution, it seems to me — and, again, not being a diplomat — that the issue is much bigger than just the nuclear issue. And so focusing on the nuclear issue by itself is not going to be able to get us there.

GWEN IFILL: Siegfried Hecker, former director of the Los Alamos Laboratory, and Wendy Sherman, many former titles, but you were former undersecretary, most recently, of political affairs at the Department of State.

(LAUGHTER)

GWEN IFILL: Thank you both very much.

WENDY SHERMAN: Thank you.

SIEGFRIED HECKER: Thank you very much. It's my pleasure.