Turkey launched a long-threatened military operation against Kurdish forces in Syria on Wednesday with a barrage of airstrikes and artillery fire, defying international outrage to open a new front in the country's war.

Turkey’s president Recep Tayyip Erdogan took to Twitter to declare the start of an operation as the first strikes hit Syrian towns and villages, saying it was aimed preventing a “terror corridor” across his country’s southern border.

The bombings sent waves of civilians fleeing from towns and villages in the region. At least seven people were killed in the initial bombardment, according to local monitors. Turkey began a ground offensive hours later, Turkish officials said.

Turkey has threatened for years to attack Kurdish forces in Syria, but the operation appears to have been set into motion just days ago, when Donald Trump unexpectedly gave Turkey the green light to launch an offensive against the Kurdish-led Syrian Democratic Forces (SDF).

Mr Trump ordered the withdrawal of US forces from the border area, and said the US would not prevent a Turkish attack against its Kurdish allies.

Syrian government offensive in Idlib Show all 8 1 /8 Syrian government offensive in Idlib Syrian government offensive in Idlib Syrians, including a member of the White Helmets civil defence rescue teams, run for cover during an air strike by pro-regime forces on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas AFP/Getty Syrian government offensive in Idlib Syrian rescue teams and civilians search for survivors amid the rubble in a freshly bombarded area following a reported air strike by regime forces and their allies on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas. AFP/Getty Syrian government offensive in Idlib Syrian rescue teams and civilians search for survivors amid the rubble in a freshly bombarded area following a reported air strike by regime forces and their allies on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas AFP/Getty Syrian government offensive in Idlib Syrian rescue teams and civilians look for survivors in a heavily damaged area following a reported air strike by regime forces and their allies on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas. AFP/Getty Syrian government offensive in Idlib This photo provided by the Syrian Civil Defense White Helmets, which has been authenticated based on its contents and other AP reporting, shows Civil Defense workers searching for victims under the rubble of a destroyed building after an airstrike by Syrian government forces, Maaret al-Numan, a town in south Idlib, Syria, Thursday, May 30, 2019. Rescue workers and activists say at least five, including three from the same family, have been killed in Syria's last rebel stronghold when warplanes targeted the building they live in, leveling it. AP Syrian government offensive in Idlib A Syrian girl cries as she runs for cover during an air strike by pro-regime forces on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas. AFP/Getty Syrian government offensive in Idlib Syrians run for cover during an air strike by pro-regime forces on the market town of Kfar Ruma in Syria's southwestern Idlib province, on May 30, 2019. - Regime forces and their Russian allies have over the past three months escalated bombardment of Syria's last major jihadist bastion in the province of Idlib and adjacent areas. AFP/Getty Syrian government offensive in Idlib A man rescues a child while members of the Syrian civil defence known as the White Helmets search the area for survivors following following a reported air strike by regime forces and their allies on Maaret al-Noman in Syria's southwestern Idlib province. AFP/Getty

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.

A spokesperson from the group called Mr Trump’s decision to withdraw a “stab in the back”.

Mr Trump’s abrupt policy shift, announced following a late night phone call with Mr Erdogan on Sunday, surprised even close allies and set into motion a new round of conflict that most US military officials had thought unavoidable. The president did not give advance notice of his policy shift to Washington’s allies in the anti-Isis coalition.

Despite calls for caution from the international community, Turkey quickly pressed ahead with preparations for a military operation.

There are now grave concerns that Turkey’s offensive could affect the security around Isis prisons and camps, home to some 18,000 militants and more than 70,000 refugees.

The SDF has said it is has had to withdraw some soldiers and redeploy them to defend against Turkey, raising fears that Isis sleeper cells could try to attack the prisons and liberate those inside.

Defending his decision on Wednesday, Mr Trump 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 on 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 the 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.

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 preparations have taken the international community by surprise, a Turkish incursion into Syria has been on the cards for some time.

The alliance between the US and the Kurds has always infuriated Ankara, which considers the SDF a terrorist organisation for its links to a 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.

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The presence of some 1,000 US 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 on 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.

The move has drawn heavy criticism from senior Republicans. Senator Lindsey Graham, a strong Trump ally, accused the president of “abandoning” the Kurds. Following the start of the Turkish bombing, Mr Graham said he would lead efforts in congress “to make Erdogan pay a heavy price”.

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.