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The United States warned its NATO ally Turkey on Monday it is "deeply concerned" after a Turkish-led assault on the Syrian city of Afrin triggered an exodus of Kurdish civilians.

Turkish forces and Turkish-backed Syrian Arab fighters have over the past 48 hours surged into the city in northwest Turkey, once defended by the Kurdish People's Protection Units (YPG) militia.

"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," State Department spokeswoman Heather 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," she said.

In eastern Syria, the YPG forms the backbone of the US-backed Syrian Democratic Forces (SDF), the militia which ousted the Islamic State group from its main stronghold of Raqqa.

But, while American special forces continue to support the SDF east of the Euphrates river, they have not come to their aid in Afrin, a pocket of autonomous Kurdish-led rule west of the river.

There, Turkish President Recep Tayyip Erdogan has unleashed his army and Syrian former rebel fighters to take Afrin from a group he sees as aligned with the outlawed Kurdish PKK group fighting within Turkey.

The fight has embarrassed America, which wants to maintain ties with both its traditional NATO ally and a regional force that has proved its mettle against IS.

Nauert said the offensive had worsened the "humanitarian situation in the area, with United Nations agencies reporting a displaced population in or from Afrin district in the hundreds of thousands."

Colonel Rob Manning, a Pentagon spokesman, said the Defense Department was concerned about reports Afrin residents no longer have access to clean water.

"We encourage all parties to allow the free flow of these resources and humanitarian aid... throughout Syria," he said.

IS 'reconstituting'

Nauert stressed that Washington is not taking sides west of the Euphrates, despite Erdogan having declared that his forces will now take the battle to more Kurdish-held districts.

"We remain committed to our NATO ally Turkey, to include their legitimate security concerns. We also remain committed to the Defeat ISIS campaign and our Syrian Democratic Forces partners in eastern Syria," she said.

Nauert also warned that the fighting has distracted from the battle against IS, which she said had begun "reconstituting in some areas."

"This is a serious and growing concern," she added.

Turkish Foreign Ministry spokesman Hami Aksoy disputed Nauert's comments.

"The claim that the operation against terrorists in Afrin would harm the fight against Daesh lacks any basis," Aksoy said, adding that "necessary precautions" have been taken to ensure civilians are not harmed and access to humanitarian aid has been secured.

The situation in Afrin has seen some Kurdish members of the SDF abandon the battle against IS in the Middle Euphrates River Valley and head west to join the fight against Turkey and Turkish aligned forces.

The Pentagon cautioned that this flow has created an opening for IS militants.

"It is imperative that we not relent on ISIS or permit these terrorists to recover from their battlefield losses," Pentagon spokesman Major Adrian Rankine-Galloway told AFP.

"There is no doubt that, as it has done in the past, ISIS is taking full advantage of any opportunity to regain its momentum by attempting to retake previously liberated territory and fleeing to more permissive areas."