MEGAN THOMPSON: The U.S. director of national intelligence estimates the number of people who've traveled from their home countries to Syria and Iraq to take up arms with the Islamic State group, ISIS, now exceeds 36,000.

These so-called foreign fighters hail from more than 80 countries, and, according to The Soufan Group, the number from Western European nations has doubled since June 2014. Two of the Brussels bombers are believed to have trained with ISIS in the Middle East.

But is this influx of foreign fighters causing ideological discord in ISIS?

Wall Street Journal reporter Matt Bradley has been reporting on that, and he joins me now by Skype from Beirut.

So, Matt, I think that most people have assumed that these foreign fighters have been welcomed with open arms by ISIS, but you have written that that's not necessarily the case. What is going on?

MATT BRADLEY, The Wall Street Journal: Well, it is and it isn't.

I mean, of course, the foreign fighters are welcomed with open arms because Islamic State wants to be able to project an image of a globally appealing Islamic ideal. But it's not quite that simple.

There is, of course, quite a lot of discord. And what is sparking the discord within — between the foreign fighters and the local fighters are the problems with money and the problems with battlefield defeats. And that's increasingly a problem for Islamic State, especially in their self-declared caliphate in Iraq and Syria.

MEGAN THOMPSON: Talk a little bit more about that and what all these conflicts are that are happening.

MATT BRADLEY: When you have foreign fighters who are rewarded for their trip, for their sacrifice, coming to the caliphate, with more money, with more spoils from war, there is bound to be resentments from the local fighters.

These are local fighters who are simply, for the most part, living under Islamic State rule. So, they have less of a choice. So, there's going to be some discord. And this is especially acute when there's battlefield defeats.

So, when they lose something like, as we're seeing right now, city of Palmyra, Islamic State doesn't necessarily have the vocabulary to accommodate a defeat. This is a group that constantly tells its followers that they are working with the writ of God. So, when they're defeated, they have to find another way of explaining that.

And, sometimes, that means executing people within their own ranks. And for the most part, that means executing some of the local fighters, not the foreigners. And this is — this projects this image of a foreign occupying force.

MEGAN THOMPSON: Do you think these internal conflicts have the potential to fracture ISIS?

MATT BRADLEY: Most of the people that I have talked to, even those who live under Islamic State rule, they say that this is going to be more of a symptom of the decline of Islamic State, rather than a cause.

I mean, it is very tempting to say that Islamic State is being torn from within because of these fractures, because of these differences between foreign fighters and local fighters. That's not really the case.

What we're talking about really is a tension that has existed within the group that's becoming more and more acute as they lose territory, as they lose funding. But we can't really talk about the demise of the group yet. We don't know. We don't have enough information about what's going on within Islamic State, and particularly within Islamic State within the borders of the caliphate in Syria and Iraq.

MEGAN THOMPSON: Matt Bradley of The Wall Street Journal, thank you so much for joining us.

MATT BRADLEY: Thank you.