"All preparations" for a military incursion into northern Syria have been "completed", Turkey's Defence Ministry has said.
Framing its decision to cross the border for an expected Turkish attack on the Kurds - who were instrumental in the defeat of so-called Islamic State - 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 statement came just hours after Donald Trump declared US troops would step aside and withdraw from the "endless war" in the Middle East, ahead of the expected Turkish incursion.
The withdrawal is underway.
Acknowledging that "the Kurds fought with us," Mr Trump claimed they "were paid massive amounts of money and equipment to do so".
Syrian Kurdish fighters, known as the Syrian Democratic Forces (SDF), called the US's move a "stab in the back" which risked gains made in the fight against the so-called Islamic State group.
They continued they would "not hesitate for a moment in defending our people" against Turkish troops, adding that it has lost 11,000 fighters in the war against IS in Syria.
A Turkish attack would lead to a resurgence of so-called Islamic State it said.
Sleeper cells of the terror group are already plotting to break free some 12,000 militants detained by Syrian Kurdish fighters in north-eastern Syria in a "threat to local and international security".
The US President was widely criticised, even from Republican allies, for what has been seen as his abandonment of the Kurds, who have fought alongside Americans for years.
In a series of tweets posted on Monday, he threatened to destroy the Turkish economy if it acted in a way he considered - in what he said was his "great and unmatched wisdom" - "off-limits".
“If Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the economy of Turkey (I’ve done before!),” he said on Twitter.
Mr Trump followed the tweets with a second flurry on Tuesday.
In the social media posts he wrote how the withdrawal is does not mean the US has "abandoned the Kurds" nor its "relationship with Turkey".
He reiterated his threat to Turkish finances, saying "any unforced or unnecessary fighting" will be devastating to its "very fragile economy".
Even Mr Trump’s staunchest Republican congressional allies expressed outrage at the prospect of abandoning Syrian Kurds who had fought the Islamic State group with American arms and advice.
“A catastrophic mistake,” said Liz Cheney of Wyoming, the No 3 House Republican leader. “Shot in the arm to the bad guys,” said Senator Lindsey Graham of South Carolina.
Mr Trump said he understood criticism from fellow Republican leaders but disagreed with them.
Pentagon and State Department officials held out the possibility of persuading Turkey to abandon its expected invasion. US officials said they had seen no indication that Turkey had begun a military operation by late on Monday.
Mr Trump appeared largely unconcerned at the prospect of Turkish forces attacking the Kurds, who include a faction he described as “natural enemies” of the Turks.
“But I have told Turkey that if they do anything outside of what we would think is humane … they could suffer the wrath of an extremely decimated economy,” he said.
In recent weeks, the US and Turkey had reached an apparent accommodation of Turkish concerns about the presence of Kurdish fighters, seen in Turkey as a threat.
American and Turkish soldiers conducted joint patrols in a zone along the border. As part of that work, barriers designed to protect the Kurds were dismantled amid assurances that Turkey would not invade.
Mr Graham said Turkey’s Nato membership should be suspended if it attacks into north-eastern Turkey, potentially annihilating Kurdish fighters who acted as a US proxy army in a five-year fight to eliminate the Islamic State’s so-called caliphate.
Mr Graham, who had talked Mr Trump out of a withdrawal from Syria last December, said letting Turkey invade would be a mistake of historic proportion and would “lead to Isis reemergence”.
Turkey has threatened for months to launch a military operation to drive away Syrian Kurdish fighters from a border region east of the Euphrates River.
The SDF captured the last sliver of land held by so-called Islamic State, marking the end of the so-called caliphate that was declared by the extemists' leader Abu Bakr al-Baghdadi in 2014.
The SDF also control the al-Hol camp, home to more than 70,000 mostly wives and children of IS fighters.
Turkish foreign minister Mevlut Cavusoglu tweeted that since the beginning of the crisis in Syria, "we have supported the territorial integrity of this country, and we will continue to support it".
He added that Ankara is determined to ensure the survival and security of Turkey "by clearing the region from terrorists. We will contribute to peace, peace and stability in Syria".