Preparations for a Turkish military operation in northern Syria are complete, Turkey's defence ministry said early on Tuesday.
It came after US President Donald Trump's decision to pull back troops paved the way for an attack on Kurdish-led forces in the region.
Kurdish forces, instrumental in the fight against the Islamic State group, have long been key allies of the United States, but Turkey regards them as terrorists due to their links to Kurdish militants waging an insurgency inside Turkey.
Facing backlash even from top Republicans over the decision, Trump said he will "totally destroy and obliterate the economy of Turkey" if the country does anything he considers "off-limits" in northern Syria.
House Speaker Nancy Pelosi, a Democrat, and Republicans including Senate Republican leader Mitch McConnell, Senator Lindsey Graham, and former ambassador to the UN Nikki Haley all blasted the move.
France's armed forces minister, Florence Parly, warned the decision to withdraw could open the door to a revival of Islamic State.
Relations have been tense between Ankara and Washington for a number of years, not the least for the US' longstanding policy in Syria, backing Kurdish forces in the fight against IS.
The White House statement signalled a significant shift in US policy — a move that could have major geopolitical ramifications.
What does the White House statement outline?
The White House press secretary Stephanie Grisham confirmed that Turkey will be going ahead with their long-planned operation in northeast Syria to create a "safe zone", and added that, crucially, the US will no longer be in the immediate area.
The statement went on to point the finger at several European countries including France and Germany, saying that many IS fighters originate from the continent.
"The United States will not hold them for what could be many years and at a great cost to the United States taxpayer," Grisham said.
The statement did not confirm the extent of this planned operation. A spokesperson for Turkey's President Erdogan refused to comment on whether an incursion was imminent.
Why will the operation focus on northern Syria?
The border between Syria and Turkey has long been a point of concern for Ankara. Kurdish forces such as the YPG and Syrian Democratic Forces (SDF) have been fighting against IS in Syria with US backing.
But its sister organisation, the PKK, the Kurdish separatist group in southern Turkey, is recognised as a terrorist organisation by Ankara, the EU, and Washington.
In 2015, a ceasefire broke down between the PKK and Turkey, with numerous insurgencies by the latter since.
And so in the eyes of Ankara, the Kurdish forces in Syria and those in southern Turkey are one of the same.
"The TSK will never tolerate the establishment of a terror corridor on our borders. All preparations for the operation have been completed," the Turkish Defence Ministry said on Twitter early on Tuesday.
"It is essential to establish a safe zone/peace corridor to contribute to our region's peace and stability, and for Syrians to achieve a safe life," it said.
Why is it seen by some as controversial?
US President Trump's policy on Syria has long been a point of contention, having announced in December that he planned a full withdrawal of troops from Syria.
That, as yet, has not occurred although his previous Secretary of Defence Jim Mattis resigned in 2018 due to those plans of withdrawal from the region.
The confirmation by the White House that President Erdogan will be going ahead with the operation has been seen as controversial for mainly three reasons.
First, some analysts have said that this move will see SDF troops move toward the northern border to meet with Turkish forces, which could leave a vacuum for ISIS to fill in other parts of Syria - areas such as Raqqa which are tentatively controlled by Kurdish forces as is.
Others have pointed to camps with suspected ISIS detainees, adding that unrest could signal an escape route for those within these holdings.
Euronews journalist Gareth Browne travelled to Ain Issa camp in August. Having spoken to detainees, Browne said on Twitter: "The impression was that a Turkish invasion of Kurdish-held areas was their best shot at getting out".
Members of Congress such as Iraq army veteran Ruben Gallego took to Twitter with concerns of the US turning their backs on their Kurdish allies who have fought IS in Syria for the past 8 years.
Gallego said that "they will look for new alliances or independence to protect themselves," adding that "allowing Turkey to move into northern Syria is one of the most destabilising moves we can do in the Middle East".
Even some top Republicans have condemned the move, most notably Lindsey Graham, who has been a steadfast supporter of Donald Trump.
He took to Twitter warning Turkey can't keep ISIS from re-emerging and insisting Turkey's primary target is the Kurds, adding Turkey's threat to invade Syria was "a major escalation of tensions between the US and Turkey".
The former US ambassador to the United Nations, Nikki Haley, warned "We must always have the backs of our allies", adding that leaving the Kurds "to die" would be a "big mistake".
The SDF issued a lengthy statement on Twitter, saying that they implemented "all of our commitments to remove military fortifications between Tell Abyad and Sari Kani, withdraw combat forces with heavy weapons, risking a security vacuum as a result of the agreement," making the Kurds more vulnerable, adding that a Turkish attack "would have bad consequences for the whole region".
Brett McGurk, a former US presidential envoy who resigned along with Mattis, took to Twitter to express his concern over the move, with Turkey set to have authority over the IS members remaining in northern Syria.
"Turkey has neither the intent, desire, nor capacity to manage 60-thousand detainees in al Hol camp, which State and DoD IGs warn is the nucleus for a resurgent ISIS. Believing otherwise is a reckless gamble with our national security," McGurk said.
How have Turkey-US relations been?
Speaking on Monday, Erdogan claimed US troops had already begun withdrawing from northern Syria following a phone call with his US counterpart the day previously, in what could be a step toward easing tensions between Washington and Ankara.
Relations between the two dropped to a freezing point with the arrest of pastor Andrew Brunson, a US citizen who was detained on alleged terrorism charges, in 2018. The US slapped sanctions on Ankara, with the Lira dropping dramatically.
Turkey released Brunson in October of last year almost simultaneously as details came forward about Washington Post journalist Jamal Khashoggi's murder in the Saudi consulate in Istanbul, many seeing this as Erdogan looking to get leverage over Riyadh.
However, in the months since, the US has continued to back Saudi Arabia in the Yemeni conflict which many see as a proxy war for power in the Middle East.
Erdogan has a planned visit to Washington in November.