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Defense Secretary Ash Carter has a blunt message for his counterparts fighting the Islamic State. The U.S. wants and expects them to do more. Carter meets with those defense chiefs tomorrow in Brussels. The summit is happening as the Syrian regime wages a bloody offensive against rebel-held Aleppo and as doubts are growing about the U.S. strategy there. Here's NPR's David Welna.

DAVID WELNA, BYLINE: This the first time Defense Secretary Carter is gathering the defense ministers from all the nations that have contributed militarily to the U.S.-led campaign against ISIS. Carter recently told NPR he expects those 26 partners to pull their own weight.

ASH CARTER: And the question I'm going to be putting to them is, what part are you going to play? The United States has indicated that it's willing to do more to hasten the necessary defeat of ISIL. What are you going to do?

WELNA: Carter has lately taken to naming and shaming coalition partners seen by the U.S. as laggards. Here's what he had to say in Davos, Switzerland, about Turkey.

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CARTER: I think the Turks can do more to fight ISIL. They're helping us fight ISIL by, for example, hosting our aircraft in Turkey. I'm grateful for that, but I think they can do more.

WELNA: Others agree. Philip Gordon, who until last year was the White House coordinator for the Middle East, says Turkey has failed to stop recruits from using its porous border to join ISIS in Syria.

PHILIP GORDON: I think, in some ways, the public criticism reflects real impatience and a real concern that, you know, we can apply all the military assets in the world, but if foreign fighters are streaming across that border into Syria, it won't be enough.

WELNA: The Pentagon chief has also recently accused the Sunni-led nations along the Arabian Gulf, which includes Saudi Arabia, of making the least contributions to the counter-ISIS effort. Here's Carter late last month on CNN.

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CARTER: We need capable local forces. The Gulf states can make contributions to galvanizing recruiting such local forces. Up to now, they haven't done enough. We want them to do more.

WELNA: Those Sunni nations may want to do more, says Middle East expert Joshua Landis of the University of Oklahoma, but their biggest concern is Shiite-aligned regime in Syria of Bashar al-Assad, not the Sunni Islamic State.

JOSHUA LANDIS: The problem that Ash Carter has is he's trying to rally the Sunnis to kill fellow Sunnis, and they don't want to do it.

WELNA: Another Middle East expert, Kenneth Pollack of the Brookings Institution, says at heart of this debate is a fundamental difference over the U.S. strategy of an air war focused exclusively on ISIS.

KENNETH POLLACK: What you regularly hear from folks in the region is that's not going to be enough. Syria and the problems of Syria are much bigger than ISIS, and you're going to have to deal with the civil war and with the Assad regime. Airpower alone - even airpower and some special forces and some weapons is never going to be enough. You're going to have to get in on the ground. America, we need you to lead the effort because we can't. But if you're willing to do so, we're willing to pony up resources to support it.

WELNA: Indeed, just a few days ago a spokesman for Saudi Arabia's military made an unexpected announcement on Al Arabiya TV.

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BRIGADIER GENERAL AHMED AL-ASIRI: (Foreign language spoken).

WELNA: Brigadier General Ahmed al-Asiri said Saudi Arabia is now willing to send ground forces to Syria, but only if coalition leaders agree to put their own forces on the ground as well. On Monday, U.S. State Department spokesman John Kirby reacted cautiously to that offer.

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JOHN KIRBY: We still just need to get a better sense of, obviously, what they envision this would entail and make sure that it matches up with coalition efforts and coalition needs.

WELNA: Because what some are now willing to give may not match with what Defense Secretary Carter wants for Syria. David Welna, NPR News, Washington. Transcript provided by NPR, Copyright NPR.