A Syrian regime soldier waves the national flag in the town of Tal Tamr as Damascus deploys forces towards the Turkish border. — AFP/VNA Photo

WASHINGTON — The US slapped sanctions on Turkey on Monday as it demanded an end to the deadly incursion against Syrian Kurdish fighters, accusing its NATO partner of putting civilians at risk and allowing the release of Islamic State extremists.

The actions came hours after Syrian regime troops returned for the first time in years to northeastern parts of the country, invited by Kurdish fighters desperate for protection as the US pulls out.

President Donald Trump took extraordinary measures against a country that is officially a US ally as he faces mounting criticism at home, where even usually supportive lawmakers accuse him of abandoning Kurds who had spearheaded the fight against the Islamic State group.

"I am fully prepared to swiftly destroy Turkey's economy if Turkish leaders continue down this dangerous and destructive path," Trump, who until recently had touted his friendship with Erdogan, said in a statement.

The Treasury Department said it was imposing sanctions on Turkey's defense, interior and energy ministers, freezing their US assets and making US transactions with them a crime.

But House Speaker Nancy Pelosi criticised the sanctions on Twitter, writing they "fall very short of reversing the humanitarian disaster brought about by (Trump's) own erratic decision-making."

Vice President Mike Pence said he would travel shortly to Turkey and that Trump had telephoned Erdogan on Tuesday to insist that Turkey end the operation. Trump said he was also ending talks on a US-Turkey trade deal he valued at US$100 billion and, in perhaps the most biting reprisal, re-imposing tariffs of 50 per cent of Turkish steel.

The US had slapped the 50 per cent sanctions on Turkey last year to win the release of an evangelical pastor whose detention had stirred up Trump's base.

Signaling an escalating rift in relations, Defense Secretary Mark Esper said he would head next week to Brussels to ask NATO allies to punish Turkey over the incursion.

NATO has long been seen as keeping Turkey in the Western orbit, but Erdogan angered the US earlier this year by buying the major S-400 missile defense system from Russia.

Erdogan has vowed to crush the Kurdish-led Syrian Democratic Forces, which he links to separatist militants inside Turkey.

Turkey wants to create a roughly 30-kilometre buffer zone along its border to keep Kurdish forces at bay and also to send back some of the 3.6 million Syrian refugees on its soil.

The chaos in areas targeted in the six-day-old Turkish assault has already led to the escape of around 800 foreign women and children linked to IS from a Kurdish-run camp, according to Kurdish authorities. — AFP