US forces have started to withdraw from parts of northern Syria to make way for a Turkish invasion in the region after Donald Trump agreed to let Turkey's President Tayyip Erdogan launch his long-planned offensive.

The pull-back of troops comes hours after the White House announced Ankara would soon move forward with its objective to create a 'safe zone' in northern Syria and that US soldiers will not support or be involved in it.

Speaking after a phone call with President Trump, Erdogan said that the operation could begin at any moment as Russia warned Syria's territorial integrity must be preserved.

Following the announcement of Washington's abrupt decision to stand aside, the UN has said it is 'preparing for the worst' in the region, fearing an assault could send civilians fleeing.

Kurdish forces warned a Turkish invasion threatened to destabilise the area, causing chaos in SDF-controlled prisons and refugee camps packed with battle-hardened jihadists, meaning ISIS terror cells could reform if a 'security vacuum' formed.

The US has already warned Kurdish fighters that they will not defend them from any Turkish attack, an unnamed American official has claimed.

US soldiers previously worked closely with Kurdish forces of the Syrian Democratic Forces (SDF), led by the YPG militia, in the battle to destroy Islamic State's caliphate.

Turkey considers the YPG to be a terrorist group and has long threatened to launch an assault on the Kurds as Ankara sees the group's growing power along its border as a threat to its security.

US military vehicles traveling down a main road in northeast Syria today. US-backed Kurdish-led forces in Syria said American troops began withdrawing from their positions along Turkey's border in northeastern Syria

An abandoned checkpoint was seen in Tel Abyad in northern Syria as US troops evacuated the border town today

The White House released a statement Sunday, saying President Trump spoke with Turkish President Recep Tayyip Erogdan by telephone to discuss the plans for Turkey to invade Northern Syria (both leaders pictured in June 2019)

But there are fears a Turkish advance in to the volatile region will reverse years of work done to drive extremists out and allow ISIS to regroup.

The US-backed SDF that controls much of the northeast region along Turkey's border, added it 'will not hesitate for a single moment' to defend itself from an expected Turkish invasion and threatened 'all-out war on the entire border'.

A US official said American forces had evacuated two observation posts at Tel Abyad and Ras al-Ayn in northeast Syria, along the Turkish border. Other US forces in the region were still in position for now, the official added.

Kurdish-led forces in northeast Syria confirmed today that US forces had withdrawn from areas at the border with Turkey where a threatened Turkish offensive would hurt its war against ISIS and roll back five years of security achievements.

A video posted by a Kurdish news agency showed a convoy of American armored vehicles apparently heading away from the border area of Tal Abyad.

Pictures also showed abandoned checkpoints in Tel Abyad and Ras al-Ayn after US troops evacuated the border towns.

Erdogan spoke hours after the White House said US forces in northeastern Syria will move aside and clear the way for an expected Turkish assault - essentially abandoning Kurdish fighters who fought alongside American forces in the years-long battle to defeat ISIS.

In a major shift in US policy, the White House released a statement late on Sunday, saying President Trump spoke with Erogdan by telephone to discuss the plans and the US will remove all of its forces from the 'immediate area'.

Russia, which has supported President Bashar al-Assad with an areal bombardment campaign on his own people, said Syria's territorial integrity must be preserved.

Peskov told reporters that Moscow was aware that Turkey shared Russia's position on Syria's territorial integrity, adding: 'We hope that our Turkish colleagues would stick to this position in all situations.'

He reiterated Moscow's stance that all foreign military forces 'with illegal presence' should leave Syria.

Turkey said it will not permit the ISIS to return to the region, amid fears its threatened offensive against a Kurdish militia in Syria could bolster the jihadists.

US military vehicles were seen driving northwards in northern Syria today, ahead of an anticipated Turkish invasion of the region that the Kurds say will overturn five years of achievements in the battle against ISIS

A US official said American forces had already evacuated two observation posts at Tel Abyad and Ras al-Ayn in northeast Syria, along the Turkish border

President Erdogan during a news conference in Ankara today before his departure for Serbia, where he said US troops have started to withdraw from positions in northern Syria

Ibrahim Kalin, a presidential spokesman, wrote on Twitter today: 'Turkey will also continue to fight against DAESH (IS) and will not allow it to return in any shape and form.'

The Kurdish-led SDF said the US withdrawal threatened to create a security vacuum that would 'reverse the successful effort to defeat ISIS'.

The European Union has called for calm in northern Syria and warned that fresh fighting there is only like to drive more people from their homes.

The US informed the commander of the Kurdish-led SDF forces in Syria on Monday morning that US forces will not defend them from Turkish attacks anywhere, according to a source.

'Turkey will soon be moving forward with its long-planned operating into Northern Syria,' the US statement reads.

'The Unites States Armed Forces will not support or be involved in the operation, and United States forces, having defeated the ISIS territorial ''Caliphate,'' will no longer be in the immediate area.'

The White House also confirmed that Turkey plans to take into custody all ISIS fighters captured over the past two years that European powers have refused to take in.

'The United States Government has pressed France, Germany, and other European nations, from which many captured ISIS fighters came, to take them back, but they did not want them and refused.'

'The United States will not hold them for what could be many years and great cost to the United States taxpayer. Turkey will now be responsible for all ISIS fighters in the area capture over the past two years in the wake of the defeat of the territorial ''Caliphate'' by the United States.'

Turkish forces artillery pieces being driven to their new positions near the border with Syria in Sanliurfa province, Turkey, yesterday

US-backed Kurdish-led forces in Syria said US troops began withdrawing today from their positions along Turkey's border in northeastern Syria, ahead of an anticipated Turkish invasion. PIctured (above are Turkish artillery moving into position yesterday)

US troops left some position in northern Syria today (pictured above an abandoned checkpoint)

Ankara said its planned 'safe zone' in northern Syria could allow up to two million Syrian refugees to return.

The safe zone 'will serve two purposes: secure Turkey's borders by eliminating terrorist elements and allow refugees to return to their homes,' Kalin said.

He said Turkey had 'no interest in occupation or changing demographics'.

There are over 3.6 million Syrian refugees in Turkey, the highest number in the world, which has become an increasing source of tension in the country.

The Syrian Observatory for Human Rights, a UK-based organisation that reports on the war, said US forces had withdrawn from an area between the towns of Tel Abyad and Ras al-Ayn.

The SDF, led by the Kurdish YPG militia, said the Turkish invasion 'will have a great negative impact' on the war against ISIS.

It said in a statement: 'Despite our efforts to avoid any military escalation with Turkey and the flexibility we have shown to move forward in establishing a mechanism for the security of the borders ...the American forces did not fulfill their commitments and withdrew their forces from the border areas with Turkey.

'Turkey is now preparing for an invasion operation of northern and eastern Syria,' added the SDF, which with US backing in recent years defeated Islamic State, across much of northern and eastern Syria.

The Turkish military operation 'will have a great negative impact on our war against the Daesh organisation and will destroy everything that has been achieved with regards to stability during the last years,' it added.

In a statement the SDF said: 'The American forces did not abide by their commitments and withdrew their forces along the border with Turkey.

Syrian Democratic Forces spokesman Mustafa Bali tweeted Saturday, before the announcement was made: 'We will not hesitate to turn any unprovoked attack by Turkey into an all-out war on the entire border to DEFEND ourselves and our people.'

The Kurds have custody of thousands of captured ISIS militants, including about 2,500 highly dangerous foreign fighters from Europe and elsewhere - their native countries have been reluctant to take them back - and about 10,000 captured fighters from Syria and Iraq.

Kurdish officials have expressed concerns of a possible breakout by ISIS prisoners in case of fighting in the area.

Syrian Kurds gather around a US armored vehicle during a demonstration against Turkish threats next to a US-led international coalition base on the outskirts of Ras al-Ain town in Syria's Hasakeh province near the Turkish yesterday

US and Turkish military forces conducting the third ground combined joint patrol inside the security mechanism area in northeast Syria last week

The Turkish and US troops start their second joint ground patrol within a planned safe zone in northern Syria along the Syrian-Turkish border, in Sanliurfa, Turkey, last month

Asked about the White House comments, Erdogan said that both Turkey and the US were working separately to see 'what steps can be taken' so that foreign fighters in prison can be repatriated.

'This is being worked on,' he said today.

Panos Moumtzis, the UN's humanitarian coordinator for Syria, speaking in Geneva today, stressed there were 'a lot of unanswered questions' about the consequences of the operation.

He added civilians must be spared in any Turkish military manoeuvres and added that the UN had seen a 'bitter history' of safe zones in places like Srebrenica.

Moumtzis was referring to the slaughter by Bosnian Serb troops of 8,000 Muslim men and boys in 1995 in a UN-declared 'safe zone' where Dutch peacekeepers were unable to protect civilians.

'We don't know what is going to happen...we are preparing for the worst, he said.

'We understand that there is going to be some kind of security zone which will be very specifically targeted to a military operation or to an area where there has to be some clearance.

'So our hope is that there will be full cooperation by all to make sure that it happens as smoothly as possible, without resulting in displacement, and ensuring protection of civilians, ensuring that the basic principles of humanity will be respected on the ground,' Moumtzis added.

He said the UN's priorities were to ensure that any prospective Turkish offensive not result in new displacements, that humanitarian access remain unhindered and that no restrictions be put in place on freedom of movement.

The UN has a contingency plan to address additional civilian suffering, but 'hopes that will not be used,' Moumtzis said.

More than 1,000 US troops are currently deployed in northeastern Syria but will no longer be present during the invasions.

The US soldiers work closely with the Kurdish YPG, which leads the Syrian Democratic Forces in the regions.

Turkey is highly likely to wait until US soldiers have withdrawn from northern Syria before launching an offensive, a senior Turkish official said today.

He added that the withdrawal of US forces from the planned area of operations could take one week and that Ankara was highly likely to wait for this in order to avoid 'any accident'.

'The Unites States Armed Forces will not support or be involved in the operation,' the statement reads

Fighters from a new border security force under the command of Syrian Democratic Forces (SDF) dance during a graduation ceremony in Hasaka, northeastern Syria, last January

On Monday, the US-backed SDF said such an operation would reverse years of successful Kurdish-led operations to defeat the Islamic State group and allow some of its surviving leaders to come out of hiding.

It also warned that a Turkish invasion would pose a threat to SDF-run prisons and informal settlements housing thousands of IS jihadists and their families.

Ankara wants to push the US-backed Syrian Kurdish forces from its border, saying that the group is a 'terrorist' offshoot of Kurdish insurgents in Turkey.

The Turkish military has twice launched offensives in Syria - against IS in 2016, and in 2018 against the People's Protection Units (YPG), the backbone of the SDF.

Long marginalised, Syria's Kurds have - beyond heavy campaigns against IS - essentially stayed out of the country's eight-year civil war, instead setting up their own institutions in areas under their control.

In the areas of Ras al-Ayn, Tal Abyad and Kobane, all bordering Turkey, Kurdish forces have dug trenches and tunnels in preparation for a Turkish offensive, the Observatory said Sunday.

The US announcement will likely be seen as a long-feared abandonment of Kurdish allies who bore the brunt of the US-led campaign against the Islamic State militants.

Bali also tweeted that his group is not expecting the US to protect northeastern Syria, adding: 'But people here are owed an explanation regarding the security mechanism deal and destruction of fortifications.'

European Commission spokeswoman Maja Kocijancic said on Monday that 'renewed armed hostilities in the northeast will not only exacerbate civilian suffering and lead to massive displacement but will also risk severely undermining current political efforts'.

Kocijancic says the EU remains committed to Syria' 'unity, sovereignty and territorial integrity' and that a long-term solution to the conflict 'will not be reached through military means but requires a genuine political transition.'

The official Twitter account of the Syrian Democratic Forces said Sunday night that it had fulfilled its commitment to the US proposal for the 'security mechanism' along the border and says that any Turkish attack will 'reverse the successful effort to defeat #ISIS

In an agreement between Ankara and Washington, joint US and Turkish aerial and ground patrols had started in a security zone that covers over 78 miles (125 kilometers) along the Turkey-Syria border between the towns of Tal Abyad and Ras al-Ayn.

The SDF had removed fortification from the areas, considered a threat by Turkey, and withdrawn with heavy weapons.

But Turkey and the US have disagreed over the depth of the zone, with Ankara seeking to also have its troops monitor a stretch of territory between 19 to 25 miles (30 and 40 kilometers deep). Erdogan has continued to threaten an attack.

The United Nations currently delivers aid to 700,000 people in the densely-populated northeast region of 1.7 million.

Speaking to reporters in Ankara before departing for a visit to Serbia, Erdogan also said he planned to visit Washington to meet with Trump in the first half of November.

He said the two leaders would discuss plans for the 'safe zone', and added that he hoped to resolve a dispute over F-35 fighter jets during his visit.

The official Twitter account of the Syrian Democratic Forces said on Sunday night that it had fulfilled its commitment to the US proposal for the 'security mechanism' along the border and says that any Turkish attack will 'reverse the successful effort to defeat #ISIS.

'But Erdogan's threats are aimed to change the security mechanism into a mechanism of death, displace our people & change the stable & secure region into a zone of conflict and permanent war,' the SDF tweeted, warning that a Turkish military invasion would make Syria 'a permanent conflict area'.

Muslim convert Jack Letts, from Oxfordshire, has been pictured gaunt and lying on the floor in an overcrowded jail in northern Syria

For months, Turkish Erdogan has threatened to launch the military assault against Kurdish forces across the border he views as a threat to his country.

Republicans and Democrats have warned that allowing the Turkish attack could lead to a massacre of the Kurds, sending a troubling message to American allies across the globe.

One of the ISIS soldiers expected to be moved to Turkish custody is British Jihadi Jack Letts, who was pictured just days ago.

Dubbed 'Jihadi Jack', he was filmed among dozens of fellow Islamic State (IS) prisoners, also in orange jumpsuits, who have been captured by Kurdish militia.

The 23-year-old Muslim convert from Oxfordshire, who declared himself an 'enemy of Britain' and fled to the Middle East to join IS, has been stripped of his UK citizenship.

His mother Sally Lane pleaded for Letts to be allowed to return and face trial in this country so that he can be rescued from the conditions in which he is being held, but the Home Office dismissed her plea.

Seeing the first images of Letts in his cell since he was taken prisoner two years ago, she told The Mail on Sunday: 'It's heart-rending to see your son like this and to feel so completely powerless.

'We have been pressing the Red Cross for months to tell us what the jail is really like, but they always refuse, saying that to release this information would jeopardize their access.'