(CNN) Abandoned by the United States and facing a deepening Turkish military offensive , Kurdish forces near the northern Syrian border have struck a deal with the Syrian government, marking a major shift in the country's eight-year war.

On Monday, Syrian troops were reportedly advancing north towards the border to confront Turkish forces, returning for the first time in years to a region where the Kurds had established relative autonomy, and further solidifying Syrian President Bashar al-Assad's grip on the country.

The agreement between Damascus and the Kurds comes as US President Donald Trump ordered the withdrawal of all remaining American forces out of northern Syria. The move signals a departure from long-standing US counter-terrorism strategy in Syria, which hinged on a close partnership with Kurdish-led forces to combat ISIS.

In discontinuing that approach, the Trump administration has effectively ceded influence in northern Syria to Assad and his allies and raised the specter of a resurgent ISIS. Over recent days, Kurdish authorities have reported the escape of hundreds of ISIS family members from a camp in northern Syria, and warned that ISIS militants held in prisons could be next to go if fighting with Turkish forces continues to escalate.

The situation began to deteriorate last week when the Trump administration ordered US troops to step aside from the border in northern Syria, effectively paving the way for Turkey to launch its offensive against the Kurds, who they regard as enemies.

The Syrian Democratic Forces (SDF) who operate in the area are led by the Kurdish People's Protection Units (YPG), which Turkey considers a terrorist organization affiliated with the separatist Kurdistan Workers' Party (PKK).

Turkish President Recep Tayyip Erdogan says the incursion into Syria is aimed at clearing a "safe zone" along the border to resettle some two million Syrian refugees currently hosted in Turkey.

But there are growing concerns over the safety of hundreds of thousands of civilians in Turkey's path, as Kurdish-controlled towns come under heavy fire, key border towns are seized and main roads are cut off.

Amid the chaos over the weekend, a grisly video began circulating on social media that appeared to show the execution-style killing of a prominent Kurdish politician, as well as her driver, members of Kurdish security forces and several civilians by Turkish-backed militants

The Free Syrian Army or FSA (also called the Syrian National Army), have denied those claims. CNN could not independently verify the video.

Assad's soldiers helping Kurdish forces

Units from the Syrian army reportedly arrived in northern towns Monday, after reaching an agreement with the Kurds to deploy troops along the entirety of Syria-Turkey border.

Syrian state media shared footage that it described as showing local residents welcoming the regime forces.

The new agreement between the Kurds and Damascus represents a new alliance in an area already saturated with infighting.

Key Kurdish factions, explained Syrian Democratic Forces (SDF): Coalition of Kurdish and Arab fighters who are allied with the US in the war against ISIS. People's Protection Units (YPG): Kurdish militia which forms the backbone of the SDF and wants an independent Kurdish state in Syria. Kurdistan Workers' Party (PKK): Kurdish separatist group linked to the YPG, considered a terrorist organization by Turkey and US.

An autonomous administration set up by the Kurds, which covers a wide swathe of north and east Syria, said Sunday that it was the Syrian government's duty to protect its borders and sovereignty.

"This agreement offers an opportunity to liberate the rest of the Syrian territories and cities occupied by the Turkish army as Afrin and other Syrian cities and towns," the statement said.

The group added on Monday that is was in the process of agreeing a "memorandum of understanding" on the protection of the border with the Russian side.

The deal means that Kurdish forces, who were considered vital US allies in the fight against ISIS, would be fighting alongside allies of President Bashar al-Assad's regime.

The Syrian government has not commented on such an agreement.

And, despite reports of the new partnership, Ankara said Monday it would move forward with its plans for the northeastern Syrian town of Manbij.

"The plan is not for Turkey to go into Manbij, the plan is for the rightful owners of the area -- the Arabs, the tribes from that area who we are in touch with, to go in," Erdogan told reporters at Istanbul Airport.

"Our approach is for them to go in and for us to provide security for them."

ISIS families escape from camp

But Turkey's incursion has been condemned for undermining the stability and security of the whole region, raising fears that the chaos it has created could give rise to an ISIS 2.0.

Turkey and the Kurds have said they would work to ensure the safety of ISIS prisons during the offensive, but as the state of play on the ground continues to shift, so too do conflicting reports over how that threat is being handled.

On Monday, Turkey claimed that a jail holding ISIS fighters was emptied by the Syrian Kurdish Democratic Union Party (PYD), which controls the area now under attack. The allegation came a day after Kurdish authorities said that 785 people affiliated with foreign ISIS fighters escaped from Ain Issa, an encampment for displaced people, as a result of the fighting. Erdogan suggested the reports were "disinformation," designed to "provoke the US and Europe."

CNN was unable to verify either of the claims.

Trump, who has warned he would "obliterate" Turkey's economy if it does not contain the ISIS threat, alleged that the SDF was intentionally releasing ISIS fighters.

"Kurds may be releasing some to get us involved. Easily recaptured by Turkey or European Nations from where many came, but they should move quickly. Big sanctions on Turkey coming! Do people really think we should go to war with NATO Member Turkey? Never ending wars will end!" Trump tweeted on Monday

But, US officials have told CNN there are no indications the SDF have intentionally released any of the 10,000-plus ISIS prisoners in their custody as part of a bid to draw international support.

Photos: The conflict in northern Syria A car burns following a car bomb explosion in Tal Abyad, a city in northern Syria near the Turkey border, on Saturday, November 23. Hide Caption 1 of 41 Photos: The conflict in northern Syria Kurdish demonstrators hurl rocks at a Turkish military vehicle on Friday, November 8, during a joint Turkish-Russian patrol near the town of Al-Muabbadah in the northeastern part of Hassakah on the Syrian border with Turkey. Hide Caption 2 of 41 Photos: The conflict in northern Syria A man stands next to a burning motorcycle at the site of a deadly car bomb explosion in Tal Abyad, Syria, on Saturday, November 2. Turkey's defense ministry blamed the Kurdish People's Protection Units and the Kurdistan Workers Party, while a group aligned with the Kurds blamed Turkey. Hide Caption 3 of 41 Photos: The conflict in northern Syria A man inspects the site of the car bomb explosion in Tal Abyad. Hide Caption 4 of 41 Photos: The conflict in northern Syria A Turkish and Russian patrol is seen near the town of Darbasiyah, Syria, on Friday, November 1. Turkey and Russia launched patrols in northeastern Syria under a deal that halted the Turkish offensive against Syrian Kurdish fighters. Hide Caption 5 of 41 Photos: The conflict in northern Syria Turkish soldiers east of Ras al-Ain watch as a tank fires on positions held by the Syrian Democratic Forces on Monday, October 28. Hide Caption 6 of 41 Photos: The conflict in northern Syria Syrian Kurds wave flags in Qamishli as they take part in a demonstration in support of the Syrian Democratic Forces on October 28. Hide Caption 7 of 41 Photos: The conflict in northern Syria Kurdish forces withdraw from an area near the Turkish border with Syria on Sunday, October 27. Hide Caption 8 of 41 Photos: The conflict in northern Syria Displaced Syrian children sit on classroom tables at a school turned into a shelter on Thursday, October 24. Hide Caption 9 of 41 Photos: The conflict in northern Syria Russian military police began patrols on part of the Syrian border on Wednesday, October 23. Hide Caption 10 of 41 Photos: The conflict in northern Syria People angry over the US withdrawal hurl potatoes at American military vehicles as they pass through Qamishli on Monday, October 21. The image was taken from video provided by the Kurdish Hawar News Agency. Hide Caption 11 of 41 Photos: The conflict in northern Syria US military vehicles drive after pulling out of their base in the northern Syrian town of Tal Tamr on Sunday, October 20. Hide Caption 12 of 41 Photos: The conflict in northern Syria A US service member looks out of an armored vehicle as troops pull out of their base in Tal Tamr. Hide Caption 13 of 41 Photos: The conflict in northern Syria Smoke billows from a fire in Ras al-Ain, Syria, on Friday, October 18. US Vice President Mike Pence announced a day earlier that he and Turkish President Recep Tayyip Erdogan had agreed to a ceasefire halting Turkey's incursion into northern Syria. The Turkish government insisted that the agreement was not a ceasefire, but only a "pause" on operations in the region. Hide Caption 14 of 41 Photos: The conflict in northern Syria A woman reacts as a man's dead body arrives at a hospital in Ras al-Ain on October 18. Hide Caption 15 of 41 Photos: The conflict in northern Syria Displaced Syrians carry their belongings at a refugee camp north of Mosul, Iraq, on Thursday, October 17. Hide Caption 16 of 41 Photos: The conflict in northern Syria Local residents cheer as Turkish army tanks are transported on the outskirts of Akcakale, Turkey, on October 17. Hide Caption 17 of 41 Photos: The conflict in northern Syria A local resident of Ras al-Ain kisses the forehead of a member of the Turkish-backed Free Syrian Army on October 17. Hide Caption 18 of 41 Photos: The conflict in northern Syria Displaced people ride a motorcycle on the outskirts of Tal Tamr, Syria, on Wednesday, October 16. Hide Caption 19 of 41 Photos: The conflict in northern Syria People in Qamishli, Syria, attend a funeral Monday, October 14, for five fighters of the Syrian Democratic Forces. Hide Caption 20 of 41 Photos: The conflict in northern Syria A person inspects damage to a building in Akcakale, Turkey, on Sunday, October 13. The building was damaged by a mortar fired from inside Syria. Hide Caption 21 of 41 Photos: The conflict in northern Syria Some fighters of the Syrian Democratic Forces are buried in Qamishli on Saturday, October 12. Hide Caption 22 of 41 Photos: The conflict in northern Syria Smoke billows from Tal Abyad, Syria, on Friday, October 11. Hide Caption 23 of 41 Photos: The conflict in northern Syria Pro-Turkish Syrian fighters drive an armored personnel carrier across the border into Syria on October 11. Hide Caption 24 of 41 Photos: The conflict in northern Syria People drive past shops in Akcakale, Turkey, that were closed by owners who feared incoming shelling from Syria. Hide Caption 25 of 41 Photos: The conflict in northern Syria Turkish-backed fighters of the Free Syrian Army head toward Tal Abyad, Syria, on Thursday, October 10. Hide Caption 26 of 41 Photos: The conflict in northern Syria Refugees arrive in Tal Tamr, Syria, after fleeing the fighting on October 10. Hide Caption 27 of 41 Photos: The conflict in northern Syria A woman holds a baby after arriving in Tal Tamr. Hide Caption 28 of 41 Photos: The conflict in northern Syria Smoke rises from Tal Abyad, Syria, on October 10. Hide Caption 29 of 41 Photos: The conflict in northern Syria People in Akcakale, Turkey, watch smoke billow from inside Syria. Hide Caption 30 of 41 Photos: The conflict in northern Syria Turkish soldiers move into Tal Abyad. Hide Caption 31 of 41 Photos: The conflict in northern Syria Smoke rises from the Syrian city of Ras al-Ain on Wednesday, October 9. Hide Caption 32 of 41 Photos: The conflict in northern Syria Civilians flee during the bombardment on October 9. Roads were gridlocked with hundreds of fleeing families. Hide Caption 33 of 41 Photos: The conflict in northern Syria Turkish troops prepare for the operation on October 9. Hide Caption 34 of 41 Photos: The conflict in northern Syria Refugees leave their homes near the Turkey-Syria border. Hide Caption 35 of 41 Photos: The conflict in northern Syria Turkish military vehicles drive through Akcakale, Turkey, near the border. Hide Caption 36 of 41 Photos: The conflict in northern Syria Turkey launched airstrikes and artillery fire across the border on October 9. Hide Caption 37 of 41 Photos: The conflict in northern Syria Residents of Akcakale cheer as a convoy of Turkish military vehicles is driven to the Syrian border just after the offensive began. Hide Caption 38 of 41 Photos: The conflict in northern Syria "Our aim is to destroy the terror corridor which is trying to be established on our southern border and to bring peace and peace to the region," Turkish President Recep Tayyip Erdogan tweeted as he announced the start of the operation. Hide Caption 39 of 41 Photos: The conflict in northern Syria Smoke is seen in Tal Abyad, Syria, on October 9. Hide Caption 40 of 41 Photos: The conflict in northern Syria Kurdish people in Ras al-Ain, Syria, wave their group's flags as they protest against a military operation on Monday, October 7. Hide Caption 41 of 41

An American official, expressing anger over the recent developments, told CNN that in his opinion US policy had "failed," and that the nation and its allies "are now facing new threats at home and abroad."

"ISIS has a second life and our geo-political allies are the ones who have the advantage," said the official, speaking candidly in a personal capacity about the withdrawal of US troops from Syria. "Russia and (the Syrian) regime will take back all of the territory and Iran has freedom of movement across the region."

Syria and Russia have both voiced their readiness to step in and take foreign ISIS fighters into their custody -- a willingness that one European intelligence official speaking to CNN says raises obvious questions.

"There is a high chance fighters or their families could attempt to come back to Europe. They could also try and retake the land of the caliphate, disappear back in to ungoverned territory to regroup or a combination of all of that. Dealing with the last two would be a major challenge without a committed ground force as you can't just use air power," the official said.

'Blood on Trump's hands'

JUST WATCHED Turkish operation in Syria threatens civilians Replay More Videos ... MUST WATCH Turkish operation in Syria threatens civilians 01:42

More than 150,000 people have been displaced from border areas around Tal Abyad and Ras al-Ain, according to the United Nations Office for the Coordination of Humanitarian Affairs (OCHA).

As roads flood with families trying to escape the offensive, criticism has been leveled at Trump for pulling troops out of the area thereby helping to provide a de facto go ahead for the Turkish attack.

Retired US four-star Marine Gen. John Allen on Sunday said , "There is blood on Trump's hands for abandoning our Kurdish allies."

The former commander of American forces in Afghanistan and former special presidential envoy for the Global Coalition to Counter ISIS under the Obama administration, told CNN the unfolding crisis in Syria was "completely foreseeable" and "the US green lighted it."

"There was no chance Erdogan would keep his promise, and full blown ethnic cleansing is underway by Turkish supported militias," he said. "This is what happens when Trump follows his instincts and because of his alignment with autocrats."

The Trump administration has insisted Turkey would have proceeded with their offensive regardless of whether US troops had remained and that the US has not deserted the Syrian Kurds.