Donald Trump has said Turkey will finish off the Islamic State in Syria as he praised the country's president Recep Tayyip Erdogan as "a man who can do it".
The US president has doubled down on his decision to withdraw US troops from the region following the departure of his Defence Secretary James Mattis over the move. In a stinging resignation letter Mr Mattis warned that America must "show respect" to its allies.
Mr Trump has defended his position, tweeting: "President Erdogan of Turkey has very strongly informed me that he will eradicate whatever is left of ISIS in Syria....and he is a man who can do it plus, Turkey is right 'next door.' Our troops are coming home!"
The US military confirmed that the order to withdraw American soldiers from Syria had been signed after Mr Trump held talks with his Turkish counterpart to negotiate the terms of the pullout.
The US currently has around 2,000 troops in the civil war-racked country, where they have been deployed to assist in a multinational fight against the Islamic State (Isil) jihadist group.
President @RT_Erdogan of Turkey has very strongly informed me that he will eradicate whatever is left of ISIS in Syria....and he is a man who can do it plus, Turkey is right “next door.” Our troops are coming home!
— Donald J. Trump (@realDonaldTrump) December 24, 2018
Turkey was a rare ally that lauded Mr Trump's decision on Syria, a country where it will now have a freer rein to target Kurdish fighters who were armed and trained by the US and played a major role in the war against Isil but are deemed terrorists by Ankara.
Mr Trump and Mr Erdogan spoke by telephone on Sunday and "agreed to ensure coordination between their countries' military, diplomatic and other officials to avoid a power vacuum which could result following any abuse of the withdrawal and transition phase in Syria," the Turkish presidency said in a statement.
Mr Trump tweeted that he and Mr Erdogan "discussed (Isil), our mutual involvement in Syria, & the slow & highly coordinated pullout of US troops from the area."
Mr Trump's advisers have reportedly persuaded the president to withdraw the troops more slowly than he would like in order not to jeopardise their safety.
A withdrawal could have extraordinary geopolitical implications, and it runs counter to long-established US policy for the region.
Thousands of Isil fighters are thought to remain in Syria, but Mr Trump on Wednesday declared that "we've won against ISIS," using another acronym for the extremists.
Late on Sunday he tweeted that President Erdogan had assured him that any remaining Isil fighters will be eliminated.
However US politicians - including those from Mr Trump's own Republican party - and international allies fear the withdrawal is premature and would further destabilise the already devastated region.
A US pullout, said Mutlu Civiroglu, a Kurdish affairs analyst, will open the way "for Turkey to start its operations against the Kurds, and a bloody war will begin."
French President Emmanuel Macron on Sunday said he "deeply regretted" Mr Trump's decision, and that "an ally must be reliable."
Israel has been careful to point out that it respects the US decision, but analysts say that beneath those public pronouncements are concerns over whether its main enemy Iran will have a freer hand.
Mr Trump's sudden decision sparked turmoil within his administration, prompting the resignation of Brett McGurk, the special envoy to the anti-Isil coalition, as well as Mr Mattis.
Mr McGurk said he could not support Mr Trump's Syria decision, saying it "left our coalition partners confused and our fighting partners bewildered."
Unlike Mr Mattis, Mr Shanahan has never served in the military and has spent most of his career in the private sector, including with aircraft giant Boeing.
Until Mr Trump finds a permanent Pentagon chief, Mr Shanahan will lead plans for US troops to leave Syria along with a significant drawdown in Afghanistan, both of which critics worry will leave war-torn regions at risk of continued and potentially heightened bloodshed.