With hostile Turkish-backed groups mobilising against them in Syria's north, and Damascus ruled by a group friendly to Ankara, Syria's main Kurdish factions are on the back foot as they seek to preserve political gains carved out during 13 years of war.

Part of a stateless ethnic group straddling Iraq, Iran, Turkey, Armenia and Syria, Kurds have so far been among the few winners of the Syrian conflict, controlling nearly a quarter of the country and leading a powerful armed group that is a key US ally in countering Islamic State.

But the power balance has tilted against them since the Islamist group Hayat Tahrir al-Sham (HTS) swept into Damascus this month, toppling President Bashar al-Assad, two analysts and a senior Western diplomat told Reuters.

The seismic change in Syria is expected to yield deeper Turkish sway just as a change of US administration is raising questions over how long Washington will keep backing the country's Kurdish-led forces.

For Turkey, the Kurdish factions represent a national security threat. Ankara views them as an extension of the Kurdistan Workers Party (PKK), which has been waging an insurgency against the Turkish state since 1984 and is deemed a terrorist group by Turkey, the United States and other powers.

The Syrian Kurdish groups "are in deep, deep trouble", said Aron Lund, a fellow at Century International, a US-based think tank.

"The balance has shifted fundamentally in Syria to the advantage of Turkey-backed or Turkey-aligned factions, and Turkey seems determined to exploit this to the fullest."

The shift has been reflected in renewed fighting for control of the north, where Turkey-backed armed groups known as the Syrian National Army (SNA) have made military advances against the Kurdish-led Syrian Democratic Forces (SDF).

Fanar al-Kait, a senior official in the Kurdish-led regional administration, told Reuters that the ouster of Assad, whose Arab nationalist Baath Party oppressed Kurds for decades, presented a chance to stitch the fragmented country back together.

He said the administration is ready for dialogue with Turkey, but the conflict in the north showed Ankara had "very bad intentions".

"This will certainly push the region towards ... a new conflict," he added.

Turkish President Tayyip Erdogan said on Friday he expected foreign states would withdraw support for Kurdish fighters following Assad's toppling, as Ankara seeks to isolate the People's Protection Units (YPG), the Kurdish militia that spearheaded the SDF alliance.

Responding to questions from Reuters, a Turkish official said the root cause of the conflict is "not Turkey's view towards the region; it is that the PKK/YPG is a terrorist organisation".

"The PKK/YPG elements must lay down their arms and leave Syria," the official said.

SDF commander Mazloum Abdi, in a Reuters interview on Thursday, acknowledged the presence of PKK fighters in Syria for the first time, saying they had helped battle Islamic State and would return home in the event a total ceasefire was agreed with Turkey. He denied any organisational ties with the PKK.

FEMINISM AND ISLAMISM

Meanwhile, in Damascus, the new leadership is showing warmth towards Ankara and indicating it wants to bring all Syria back under central authority - a potential challenge to the decentralisation Kurds favour.

While Turkey provides direct backing to the SNA, it along with other states deems HTS a terrorist group because of its al Qaeda past.

Despite this, Ankara is believed to have significant sway over the group. A senior Western diplomat said: "The Turks can clearly influence them more than anyone else".

HTS leader Ahmed al-Sharaa told a Turkish newspaper that Assad's ouster was "not only the victory of the Syrian people, but also the Turkish people".

The Turkish official said HTS was not and never had been under Ankara's control, calling it a structure "we were communicating with due to circumstances" and adding many Western states were also doing so.

Syrian Kurdish groups led by the Democratic Union Party (PYD) and the affiliated YPG militia took control of much of the north after the uprising against Assad began in 2011. They established their own administration, while insisting their aim was autonomy, not independence.

Their politics, emphasising socialism and feminism, differ starkly from HTS' Islamism.

Their area grew as US-led forces partnered with the SDF in the campaign against Islamic State, capturing Arab-majority areas.

The Turkey-backed SNA groups stepped up their campaign against the SDF as Assad was being toppled, seizing the city of Manbij on Dec. 9

Washington brokered a ceasefire, but the SDF has said Turkey and its allies have not abided by it, and a Turkish defence ministry official said there was no such deal.

US support for the SDF has been a point of tension with its NATO ally, Turkey. Washington views the SDF as a key partner in countering Islamic State, which Secretary of State Antony Blinken has warned will try to use this period to re-establish capabilities in Syria. The SDF is still guarding tens of thousands of detainees linked to the militant group.

Turkish Defence Minister Yasar Guler said last weekend that Turkey saw no sign of an Islamic State resurgence in Syria. On Friday, Turkey's foreign minister, Hakan Fidan, told his German counterpart during talks in Ankara that alternatives needed to be found for the management of camps and prisons where the detainees are being held.

Separately, US Assistant Secretary of State for Near Eastern Affairs Barbara Leaf said on Friday that Washington was working with Ankara and the SDF to find "a managed transition in terms of SDF's role in that part of the country."

President Joe Biden's administration has said that US troops will stay on in Syria, but President-elect Donald Trump could remove them when he takes office on Jan. 20.

LETTER TO TRUMP

During his first administration, Trump attempted to pull out of Syria but faced pressure at home and from US allies.

In a Dec. 17 letter to Trump, reviewed by Reuters, top Syrian Kurdish official Ilham Ahmed said Turkey was preparing to invade the northeast before he takes office.

Turkey's plan "threatens to undo years of progress in securing stability and fighting terrorism", she wrote. "We believe you have the power to prevent this catastrophe."

Trump's transition team did not immediately respond to a request for comment.

Trump said on Dec. 16 that Turkey will "hold the key" to what happens in Syria but has not announced his plans for US forces stationed there.

"The Kurds are in an unenviable position," said Joshua Landis, a Syria expert at the University of Oklahoma. "Once Damascus consolidates its power, it will move on the region. The US can't remain there forever."

HTS leader Sharaa told British broadcaster the BBC that Kurds were "part of our people" and "there should be no division of Syria", adding arms should be entirely in the state's hands.

Sharaa acknowledged one of Turkey's main concerns - the presence of non-Syrian Kurdish fighters in Syria - and said: "We do not accept that Syrian lands threaten and destabilize Turkey or other places."

He pledged to work through dialogue and negotiations to find "a peaceful formula to solve the problem," saying he believed initial contacts had been established "between the Kurds in northeastern Syria or the SDF organisation".

Kait, the Kurdish official, said his administration wanted "a democratic Syria, a decentralized Syria, a Syria that represents all Syrians of all sects, religions and ethnicities," describing these as red lines. The SDF would be "a nucleus of the coming Syrian army," he added.

SDF commander Abdi, in his Reuters interview, confirmed that contact had been established with HTS to avoid clashes between their forces but said Ankara would try to drive a wedge between Damascus and the Kurdish-led administration.

Still, he said there was strong support from international parties, including the US-led coalition, for the SDF joining "the new political phase" in Damascus, calling it "a great opportunity".

"We are preparing, after a total ceasefire between us and between Turkey and the affiliated factions, to join this phase," he said.