Turkey renewed threats Monday to attack a town in northeastern Syria where a small contingent of U.S. troops is based, raising the stunning possibility of a clash between NATO allies.

Turkish President Recep Tayyip Erdogan said his forces and allied Free Syrian Army militias would press on with attacks against Manbij and several other northeastern Syrian towns held by the U.S.-backed Syrian Democratic Forces "until we completely abolish this [terrorist] corridor."

Erdogan spoke after his troops captured the border town of Afrin, which had been held by the Kurdish fighters of the People's Protection Units (YPG), which is the dominant force in the SDF.

"Many of the terrorists had turned tail and run away already," he said in a speech. "In Afrin's center, it is no longer the rags of the terror organization that are waving but rather the symbols of peace and security."

Monitoring groups charged that hundreds of civilians were killed and tens of thousands of refugees fled in the taking of Afrin in what Turkey is calling Operation Olive Branch.

"Civilian casualties, not getting humanitarian assistance, a growing humanitarian crisis -- it's got to be stopped, it's got to be averted," said Army Col. Rob Manning, a Pentagon spokesman.

At a Pentagon briefing, he declined to cite Turkey as being responsible but said, "More than who is to blame, is the need to stop it, and the need to ensure that all parties focus on the reason why they are there," which is the defeat of the Islamic State of Iraq and Syria (ISIS).

However, Manning indicated that the U.S. would not leave Manbij or abandon support for the SDF.

"We've made that very clear that U.S. forces are in Manbij, and again, this goes back to the need to focus on the defeat of ISIS," he said.

"It's been very clear to all parties that U.S. forces are there, and we'll take measures to make sure that we de-conflict," Manning said.

At the State Department, spokesperson Heather Nauert said the U.S. is "deeply concerned over reports from Afrin city over the last 48 hours."

"It appears the majority of the population of the city, which is predominantly Kurdish, evacuated under threat of attack from Turkish military forces and Turkish-backed opposition forces," she said.

In highly unusual remarks aimed at an ally, Nauert said, "We are also concerned over reports of looting inside the city of Afrin. We have repeatedly expressed our serious concern to Turkish officials regarding the situation in Afrin."

The split with Turkey on U.S. support for the SDF had been building for months and intensified when Turkey pushed across the border into Syria on Jan. 20 in Operation Olive Branch.

Turkey considers the YPG fighters of the SDF to be terrorists allied with the PKK (Kurdish Workers Party), which has been labeled a terrorist organization by Turkey and the U.S.

However, the YPG played the leading role in retaking Manbij from ISIS, which set the stage for the major victory of the war against ISIS in Syria -- the taking of the so-called capital of the "caliphate" in Raqqa.

Last week, the U.S. announced a "pause" in ground operations against ISIS in the middle Euphrates River Valley (MERV) as YPG fighters began leaving their posts to travel north to battle the Turkish forces.

-- Richard Sisk can be reached at Richard.Sisk@Military.com.