Chloe Cornish in Beirut, Laura Pitel in Ankara and Henry Foy in Moscow

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Just a week after Turkey launched a blistering military offensive into north-east Syria, the Assad regime Ankara tried to help topple is emerging as one of the biggest winners.

On Monday, President Bashar al-Assad’s troops were rolling back into a region the regime quit seven years ago, after the Kurdish militants being targeted by the Turkish air and land assault were forced to strike a deal with Damascus.

It means the door has been opened for the government to potentially reassert control over another oil-rich chunk of the country, and it is now partnering with the Kurdish-dominated Syrian Democratic Forces that were armed and trained by Washington.

The fast-moving events have suddenly redrawn the lines of control in the strategic Syrian-Turkish border area. The deal between the Assad regime and the SDF was announced on Sunday hours after the US said it was withdrawing its remaining forces from Syria, in a move analysts say will please Russia and Iran, which back Damascus, but unnerve Ankara, which has long supported anti-Assad militants.

“People in Damascus are celebrating,” said Emile Hokayem of the International Institute for Strategic Studies think-tank. “It’s not just a net victory for the regime, it’s a massive one.”

People here are living in a state of anxiety and fear of what’s coming next. I think most prefer [Turkish-backed anti-Assad extremist rebels] over the regime

The SDF, whose backbone, the People’s Protection Units (YPG), is viewed by Turkey as a terrorist group, had been shielded from a threatened Turkish offensive by the US troops.

But without Washington cover, Kurdish forces turned to Moscow to help broker an agreement with the regime. “The choice between resisting alone and being exterminated, or reconciling with the regime and Russia, is clear,” said Nawaf Xelil, director for the Kurdish Studies Center think-tank in Germany.

“The worst agreement is still better than having our regions — which we have been protecting for years, and where there are a million displaced Syrians — be destroyed.”

The US withdrawal and the deal with Damascus change the calculus for foreign powers in the theatre of Syria’s civil war, now in its ninth year.

For US-led western allies, who partnered Kurdish forces in 2016 to combat Isis jihadis overrunning north-east Syria, the worst-case scenario is an Isis resurgence. Analysts say the extremist Islamist group has already stepped up attacks, taking advantage of the chaos.

Sam Heller, an analyst at the International Crisis Group, said the risk now was “a disorderly change of control” of the area “from SDF to regime”. If the handover is smooth, Isis sleeper cells may be unable to escalate their insurgency.

However, it is far from clear whether Damascus can handle prisons stuffed full of Isis suspects and vast camps housing their families, thousands of whom are foreign.

The Syrian army’s deployment along the borders brings huge uncertainty for Turkey.

Recep Tayyip Erdogan, Turkey’s president, appears to have succeeded in his aim of disrupting efforts by Kurdish forces to establish a semi-autonomous state on its doorstep.

But Mr Erdogan’s plan to resettle millions of refugees in the region, to quell deep domestic discontent over the 3.6m Syrians who have fled to Turkey, by establishing a 480km long “safe zone”, now seems in jeopardy.

Murat Yesiltas, security studies director at Seta, a think-tank closely linked to the Turkish government, said it was unlikely Turkey favoured renewed Syrian regime control over the region, which would strengthen the hand of Damascus. But he predicted this would not completely halt Mr Erdogan’s offensive, given the Turkish president’s “decisiveness” and the limited capacity of regime forces.

Mr Erdogan on Monday vowed to “finish what we started” in the region.

Under the deal, regime forces would only defend against Turkey’s advance further into Syria rather than ending Kurdish control, said SDF officials. They added that Kurdish institutions would remain.

A real understanding between the Kurds and the Syrian state would worry Ankara. Officials will be conscious of the risk of Mr Assad copying his father, former president Hafez al-Assad, by weaponising Kurdish militias against Turkey.

“The Syrian regime considers Turkey the biggest supporter of the Syrian armed opposition,” said Behlul Ozkan, associate professor of international relations at Istanbul’s Marmara University. “In the coming years, Damascus and Assad might use [Kurdish militias] as an instrument against Turkey.”

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But Mr Assad has consistently said the country’s civil war will not be over until Damascus retakes “every inch” of Syria. And while the SDF could bargain with territory that includes the lion’s share of Syria’s oil wealth, analysts say the regime has the upper hand in negotiations with the Kurdish forces, given the existential threat from Ankara.

Russia has seized on Turkey’s assault as an opportunity to reprise its role as a broker between the Assad regime, which the Kremlin has lavished with military support since 2016, and groups Damascus wants to quell in Syria.

While the Kremlin must placate its strategic ally in Ankara, and on Monday firmly rejected suggestions that the new alliances could pitch it into conflict with Turkey, the Kurdish agreement with Damascus increases Moscow’s leverage as it pushes for a post-conflict political settlement.

President Vladimir Putin of Russia visited Riyadh on Monday and will visit Abu Dhabi on Tuesday, and on Sunday called for Syria to be readmitted to the Arab League following its suspension in 2011.

For many Syrians in the region who have survived years of civil war, Mr Assad’s return raises new worries. Many living in SDF-controlled areas have openly opposed the regime and are at risk from its fearsome security apparatus, while men of fighting age could face military conscription.

“People here are living in a state of anxiety and fear of what’s coming next,” said a teacher in Arab-majority Manbij city, long a flashpoint for competition over control of the north-east. “I think most prefer [Turkish-backed anti-Assad extremist rebels] over the regime.”

Additional reporting by Asmaa al-Omar