The Turkish Defence Ministry has said that all preparations are completed for a military incursion into northern Syria.
Framing the operation as humanitarian, the ministry said in a tweet: "The establishment of a Safe Zone/ Peace Corridor is essential for Syrians to have a safe life by contributing to the stability and peace of our region."
"The Turkish Armed Forces will never tolerate the creation of a terror corridor at our borders. All preparations for the operation have been completed."
The announcement came 24 hours after US President Donald Trump revealed that American forces would pull out of an area of northern Syria where the expected incursion is due to happen.
The White House decision amounts to an abandonment of Syrian-Kurdish fighters who are long-standing US allies in the fight against Islamic State (Isis) and it exposes the Kurds to attack by Turkey.
Mr Trump's move was met with near universal astonishment and confusion even from the president's own side.
The former US Ambassador to the UN, Nikki Haley, tweeted: "We must always have the backs of our allies, if we expect them to have our back. The Kurds were instrumental in our successful fight against ISIS in Syria. Leaving them to die is a big mistake. #TurkeyIsNotOurFriend."
And Republican Senator Lindsey Graham told Fox News it was "impulsive".
"I hope I'm making myself clear how shortsighted & irresponsible this decision is in my view. This to me is just unnerving to its core."
Mr Trump tweeted an explanation, saying: "We may be in the process of leaving Syria, but in no way have we Abandoned the Kurds, who are special people and wonderful fighters. Likewise our relationship with Turkey, a NATO and Trading partner, has been very good.
"Turkey already has a large Kurdish population and fully understands that while we only had 50 soldiers remaining in that section of Syria, and they have been removed, any unforced or unnecessary fighting by Turkey will be devastating to their economy and to their very fragile currency. We are helping the Kurds financially/weapons!"
He has also said he will meet with Turkish president Recep Tayyip Erdogan on 13 November at the White House.
Overnight, a senior US Administration Official said that the White House announcement did not mean that US forces would leave Syria altogether. Only 50-100 "special operators" would be removed from a zone the Turks are preparing to move into.
The official said: "The President has made it very clear, publicly and privately, that the United States does not endorse or support any Turkish operation in Northern Syria. There will be no US Armed Forces involvement or support of any operation that the Turks undertake. We also made it clear that if Turkey undertakes such an operation, that US troops cannot be put into any danger."
The official denied that the White House decision amounted to a "green light" for Turkey to invade Syria and kill Kurdish troops.
"[President Trump] sent out a tweet today saying that if there is some sort of attack or massacre of the Kurdish people, that he is prepared, I think in his words he said, to 'obliterate the economy of Turkey'," the official said.
Speaking to Sky News from the Syrian side of the border, Aldar Khalil, a senior Syrian-Kurdish political leader, said the move would jeopardise security in multiple ways.
"It will affect the job of the local autonomous forces and the situation of ISIS prisoners being held here. Syrian regime and Russia are also preparing to attack some areas in the south and west while other military groups want to take advantage of the weakness of the local forces to capture ISIS prisoners in our region and use them to threaten Europe," he said.
Up to 70,000 Isis prisoners and their families are being held by Kurdish forces in camps near the area the Turks are preparing to advance on.
Many of those held are from western countries including the UK. Many governments are refusing to repatriate them leaving them instead in the hands of the Kurdish forces.
"Of course this decision will revive ISIS and will lead the region into a hill war and to tough conditions. A very difficult time awaits the people of the region," Mr Khalil told Sky News.
Reacting to Mr Trump's decision and the Turkish threats, the United Nations coordinator for the region said the organisation was "preparing for the worst".
For years, the Turkish President Recep Tayyip Erdogan has threatened to mount an incursion into northern Syria, which he sees as strategically advantageous.
As well as gaining land (he claims it's temporary but this could become one of those map-redrawing moments), he would also hit a long-standing Kurdish enemy.
Mr Erdogan also argues that the move would relieve the pressure his country and Europe is facing from Syrian refugees.
His government is proposing that the captured land is used to home Syrian migrants currently in Turkey and with the safety and prospects of Europe in their sights.
However, to take them back to Syria could expose them to what's left of the Islamic State as well as Syrian government forces who they escaped and who still have influence in the area.
Mr Trump's move could be seen as an attempt to call Turkey's bluff.
By pulling US forces from the vicinity and effectively leaving Turkey to mount an incursion alone, the motive could be an attempt to spook Turkey into abandoning its plans.
The move by Mr Trump also helps to fulfil his agenda domestically. He has been impatient in wanting to pull all US troops out of Syria, as part of his "America First" policy and retreat from the global stage.
"It is time now for others in the region, some of great wealth, to protect their own territory. THE USA IS GREAT!" he tweeted last night.