Turkish President Recep Tayyip Erdogan has confirmed his troops will carry out a land invasion into Kurdish areas of northern Syria.
The threat of attack has put on hold local counter-terror operations against Islamic State who are still plotting against the west.
Turkey has launched air strikes on suspected Kurdish militant targets in northern Syria and Iraq recently in revenge for a November 13 bombing in Istanbul.
As well as freeing up ISIS fighters it also puts the safety of thousands of detained ISIS refugees - such as exiled former Brit Shamima Begum - at risk.
A UK security source said: “Turkey’s threats against the Kurdish troops ultimately frees up ISIS to plot further attacks, particularly against western countries like the UK.
“It will have far-reaching consequences.”
Mr Erdogan said the air operations are "just the beginning", and that Turkey is determined to "close down all of our southern borders... with a security strip that will prevent the possibility of attacks on our country.”
Turkey has carried out a series of incursions into Syria since 2016 and already controls parts of northern Syria.
Mr Erdogan added:
"The day is near when those concrete tunnels which the terrorists use for safety will become their graves.”
The commander of the Kurdish-led Syrian Democratic Forces (SDF) in north-east Syria, meanwhile, said his group is prepared to repel a ground invasion by Turkey.
It was the British and US-trained SDF that beat Islamic State in ferocious battles and continue to be allies to western special forces.
SDF head Mazloum Abdi vowed: “We believe that we have reached a level where we can foil any new attack. At least the Turks will not be able to occupy more of our areas and there will be a great battle".
He added, "If Turkey attacks any region, the war will spread to all regions... and everyone will be hurt by that."
Mr Abdi called on Moscow and Damascus, as well as on the US-led coalition fighting against ISIS to take a stronger stance to prevent a Turkish ground invasion.
He added:
"We can say that our work against IS with the international coalition has stopped, because we are preoccupied with the Turkish attacks.
"Our coordination and work with the Russians on the ground has also been affected by the Turkish attacks."