Turkish fighter jets bombed the Syrian town of Ras al-Ayn on Wednesday as the country's president announced that a long planned offensive against Kurdish fighters had begun.
Turkey's president Recep Tayyip Erdogan said the operation was aimed at preventing the creation of a "terror corridor across our southern border."
"Operation Peace Spring will neutralize terror threats against Turkey and lead to the establishment of a safe zone, facilitating the return of Syrian refugees to their homes. We will preserve Syria’s territorial integrity and liberate local communities from terrorists," he wrote on Twitter, as witnesses reported hearing several large explosions in the border town.
The start of the operation comes just days after Donald Trump gave Turkey the green light to launch an offensive against the Kurdish-led Syrian Democratic Forces, and ordered the withdrawal US forces from the border area.
Mr Trump's abrupt policy shift, announced following a phone call with Mr Erdogan on Sunday, surprised close allies and provoked a backlash from senior Republicans. Senator Lindsey Graham, a strong Trump ally, accused the president of "abandoning" the Kurds. Senate majority leader Mitch McConnell said the withdrawal would "increase the risk that Isis and other terrorist groups regroup."
Despite calls for caution from the international community, Turkey quickly pressed ahead with preparations for a military operation.
Mustafa Bali, a spokesman for the SDF, confirmed that Turkish warplanes had begun to carry out airstrikes. "There is a huge panic among people of the region," he said on Twitter.
Witnesses reported a heavy build-up of Turkish military forces at the border ahead of the airstrikes, including a convoy of 120 vehicles carrying tanks, artillery and armoured personnel vehicles.
Mr Trump has taken to Twitter several times over the past few days to defend himself from the backlash to his decision. On Wednesday, he said that "fighting between various groups that has been going on for hundreds of years. USA should never have been in Middle East."
"Moved our 50 soldiers out. Turkey MUST take over captured ISIS fighters that Europe refused to have returned. The stupid endless wars, for us, are ending!" he wrote.
As the Turkish military amassed on the other side of the border, the Syrian Democratic Forces called on the US-led anti-Isis coalition to implement a no-fly zone over northeast Syria.
“To protect people of NE Syria from an imminent humanitarian crisis, we call the coalition and the international community for the implementation of a No Fly Zone as was done in the past for the people of Iraq,” said a statement from SDF’s official Twitter account.
The civilian administration affiliated with the group called for a "general mobilisation" along the border with Turkey.
"We call upon our people, of all ethnic groups, to move toward areas close to the border with Turkey to carry out acts of resistance during this sensitive historical time," said the local authority, known as the Autonomous Administration of North and East Syria.
The SDF earlier called Mr Trump's decision to withdraw a “stab in the back”, and said that it had assurances from the US that it would not allow any Turkish military operations against it.
It remains unclear exactly how far the Turkish operation will go. Ankara says it aims to implement a “safe zone” some 20 miles deep into Syria, stretching from the Euphrates River to the eastern border with Iraq.
Western diplomats have told The Independent that they expect a Turkish operation may at first be limited to a smaller geographical area between the towns of Tal Abyad and Ras al-Ayn. But some believe Mr Erdogan may go further.
Though the speed of Turkey's preparation have taken the international community by surprise, a Turkish incursion into Syria has been on the cards for some time.
The SDF, a mostly Kurdish militia with some smaller Arab groups attached, has been Washington’s main ally in the fight against Isis in Syria. The group received US arms, as well as significant air and ground support in the battle to defeat the Isis caliphate. It lost some 11,000 fighters in the years-long war before recapturing the last piece of Isis territory in March.
But the alliance has always infuriated Ankara, which considers the SDF a terrorist organisation for its links to banned Kurdish separatist organisation inside Turkey. It has pressured the US to drop its commitment to the group and allow it to take responsibility for ensuring the complete defeat of Isis.
The presence of some 1,000 troops in northeast Syria, working alongside the SDF, had acted as a deterrent for any Turkish attack. Mr Trump's decision to move those troops away from the border and make it clear that the US would not stand in the way of a Turkish offensive has accelerated Turkey's plans.
US allies were apparently caught off-guard by Mr Trump's announcement. British officials told The Independent that they had no advance warning of the withdrawal. Coalition forces on the ground also said they heard about the policy change in the news.
The British government said Tuesday that it opposed any Turkish operation inside Syria, and denied a claim by Mr Trump that the UK was "thrilled" with the president's decision.
“We have been consistently clear with Turkey that unilateral military action must be avoided as it would destabilise the region and threaten efforts to secure the lasting defeat of Daesh [Isis],” foreign office minister Andrew Murrison told parliament.
Iran's president Hassan Rouhani also called on Turkey to show restraint.
"We have openly said that the only solution to ensure safety and security in southern Turkey and northern Syria is the presence of the Syrian army," state news agency IRNA quoted him as saying on Wednesday.
"We are calling on our friendly and brotherly neighbour Turkey to act with more patience and restraint, and to revise its decision and chosen path," he said.
Video from near the border on Wednesday showed large tents being erected to deal with an anticipated wave of displacement from the fighting.