MARY LOUISE KELLY, HOST:

Let's go now to Syria - to northern Syria near the border with Turkey where an exceedingly complicated battlefield is taking shape. Here are the players. U.S. troops are there. They are backing local forces - Syrian Kurds - in the fight against ISIS. Meanwhile, Turkish forces have their own military offensive underway against the Syrian Kurds that the U.S. is backing. This is a problem, American commanders say, among other reasons because the U.S. and Turkey are supposed to be allies in the fight against ISIS.

Well, NPR's Tom Bowman is there on the ground bearing witness to all of this. He joins us now from an American base in northern Syria. Hey, Tom.

TOM BOWMAN, BYLINE: Hey, Mary Louise.

KELLY: I know you have been out and about all day today. Can you tell me a little bit about exactly where you were, what you saw?

BOWMAN: Well, we were in Manbij - just outside of Manbij, a city in northern Syria. Syrian Kurdish rebels recaptured this city from ISIS 16 months ago. And as you say, there's a new problem now. Turkey is fighting Kurdish rebels just to the west of here, and they're threatening to head this way. And we were at a checkpoint today, talking with Syrian Kurds and also American officers. And here's my report.

HUSSEIN AL-IBRAHIM AL-HEKMAH: (Speaking Arabic).

BOWMAN: Hussein al-Ibrahim al-Hekmah is a Kurdish rebel soldier. He's just 20, and he's peering over his fighting position outside the northern Syrian city of Manbij. His position is nothing more than a collection of sandbags topped with a piece of corrugated metal. Off in the distance, just up a dry slope covered in olive trees is a Turkish-backed Syrian rebel fighting position - just a pile of dirt, a pale chalk berm to the naked eye. He speaks through an interpreter.

AL-HEKMAH: (Through interpreter) The bullets fly over us.

BOWMAN: How often?

AL-HEKMAH: (Through interpreter) A few times a week.

BOWMAN: A stone's throw away is a two-story concrete building. It's an observation post for Syrian Kurdish rebels and their American allies. We go take a look. One of them searches through binoculars to the west. He says this position will occasionally take fire. He doesn't want his name used.

So it's coming from that village in the distance - what? - about a mile away.

UNIDENTIFIED MAN: Yes, Sir. A couple of the buildings that were up in this village...

BOWMAN: Only a couple hundred yards away.

UNIDENTIFIED MAN: Yes, Sir.

BOWMAN: Make you nervous?

UNIDENTIFIED MAN: Not really, Sir (laughter).

BOWMAN: Someone who is worried is his boss, Lieutenant General Paul Funk. He's in charge of the anti-ISIS fight in both Syria and Iraq, and he's visiting today.

LIEUTENANT GENERAL PAUL FUNK: Well, anytime our soldiers are threatened, it worries me. But they're pretty good at what they do, and they will defend themselves.

BOWMAN: So if there's a serious threat, they'll shoot back.

FUNK: Yes, they will.

BOWMAN: No American-backed Syrian rebel has been hit, nor have any of the dozens of American soldiers heading to these checkpoints daily, making sure the Turkish-supported rebels don't attack. General Funk says the American soldiers will stay even though Turkey wants the U.S. out so they can clear the entire region of Syrian Kurds. Why - because Turkey sees all these Kurds as terrorists, part of the separatist Kurdish Workers Party in Turkey that the government has declared a terrorist group. And they're fighting them just 50 miles to the west of here in an area called Afrin.

But here's the problem. The Americans say these Syrian Kurdish rebels are the best fighters against ISIS, and they've routed the Islamic State all down the Euphrates River Valley. Now hundreds of these Syrian Kurds are heading to Afrin to fight the Turks and not the Islamic State. General Funk acknowledges that it's slowing down progress against ISIS.

FUNK: Yes, it's slowing down, but it's not stopping. Our distraction and causing ourselves to have to look in multiple directions when our focus should be like a laser beam on ISIS is not helpful.

BOWMAN: As you can hear, Mary Louise, General Funk is worried about some sort of a fight between the Syrian Kurdish rebels and their Turkish-backed rebels that could even affect American troops.

KELLY: Well, Tom Bowman, what is being done to try to avert this possibility of, you know, a possible confrontation where you have Turkish-backed forces and U.S.-backed forces, who are supposed to be allies, firing at each other?

BOWMAN: Well, one thing that's going on is national security adviser H.R. McMaster is headed to Turkey this weekend to talk about this problem, and he's going to be followed next week by Secretary of State Rex Tillerson. And I think they'll echo what General Funk has said. Let's all focus on ISIS, and let's find a way to make the Turks feel a bit more secure. But right now, it's very, very tense.

KELLY: That's NPR's Tom Bowman reporting from northern Syria. Thank you, Tom.

BOWMAN: You're welcome.

ARI SHAPIRO, HOST:

And a sign of the complex battlefield there - the Pentagon says U.S. troops and their local Syrian allies were attacked elsewhere in Syria by pro-regime forces. It says there were no U.S. casualties. Transcript provided by NPR, Copyright NPR.