Video report by ITV News Correspondent Emma Murphy
Donald Trump has announced sanctions against Turkey in a bid to restrain its assault on Kurdish fighters and civilians - a week after the US President ordered American troops in northern Syria to step aside to enable an attack by Ankara.
Also on Monday it was announced that Vice-President Mike Pence is to head to the Middle East to lead mediation efforts.
Mr Pence said the US President spoke to his Turkish counterpart Recep Tayyip Erdogan earlier in the day and called for an immediate end to Turkey's attacks, which began last week, against Syrian Kurdish fighters.
Meanwhile, the American troops were scrambling for Syria's exits, a move criticised at home and abroad as opening the door to a resurgence of the terrorist organisation.
The US had been allies of Syrian Kurdish fighters in the battle against so-called Islamic State.
On Monday, Syrian government troops moved north towards the border region, setting up a potential clash with Turkish-led forces.
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Kurdish forces previously allied with the US said they had reached a deal with President Bashar Assad's government to help them fend off Turkey's invasion.
In Washington, Mr Trump said in a statement that he was halting trade negotiations with Turkey and raising steel tariffs. He said he would soon sign an order permitting sanctions to be imposed on current and former Turkish officials.
American troops consolidated their positions in northern Syria on Monday and prepared to evacuate equipment in advance of a full withdrawal, a US defence official said.
The hurried preparations, triggered by Mr Trump's decision on Saturday to expand a limited troop pullout into a complete withdrawal, came as his national security team considered imposing what he called "big sanctions" on Nato ally Turkey.
The US pullout raised many questions, including how and whether the Trump administration could continue putting military pressure on IS in Syria without a troop presence on the ground.
US forces have been there since 2015, arming and advising a Kurdish-led Syrian group of fighters who largely eliminated IS control of Syrian territory but were still working to prevent an IS resurgence.
The defence source said US officials were weighing options for the future of a counter-IS campaign, including the possibility of waging it with a combination of air power and special operations forces based outside Syria, perhaps in Iraq.
Defence secretary Mark Esper said on Sunday that Mr Trump had directed US troops in northern Syria to begin pulling out "as safely and quickly as possible". He did not say the president had ordered troops to leave Syria, but that seemed like the next step in a combat zone growing more unstable by the hour.
The only exception, it appeared, is a group of about 200 US troops who will remain at a base in southern Syria near the Jordan border, working with opposition forces unrelated to the Kurdish-led fighters in northern Syria.
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Mr Esper said the US withdrawal would be done carefully to protect troops and ensure no US equipment is left behind. He declined to say how long that might take.
In a series of tweets on Monday, Mr Trump defended his gamble that pulling US forces out of Syria would not weaken US security and credibility.
He wrote that IS prisoners who escaped amid the pandemonium in Syria can be "easily recaptured" by Turkey or European nations, even as France said it was pulling its remaining troops out of Syria.
Mr Trump took sarcastic swipes at critics who say his Syria withdrawal amounts to a betrayal of the Kurds and plays into the hands of Russia.
"Anyone who wants to assist Syria in protecting the Kurds is good with me, whether it is Russia, China, or Napoleon Bonaparte," he wrote. "I hope they all do great, we are 7,000 miles away!"