The Syrian city of Manbij, just 30 kilometres (18 miles) from the Turkish border, is controlled by Kurdish-led forces
Beirut (AFP) - After 18 months of threats, Turkey's president warned Tuesday that his troops could attack Kurdish-held areas of northeast Syria "very soon".
Why would Turkey lead such a cross-border incursion, what do Syria's Kurds want, and how is the United States -- an ally of both sides -- trying to ease tensions?
Why the Turkish threats?
While Syria's Kurds have largely stayed out of their country's eight-year war, they have set up their own institutions in a semi-autonomous region in its northeast.
Across the border, Turkey has eyed this push for increased independence with suspicion, regarding Kurdish fighters there as "terrorists".
Ankara views the Kurdish People's Protection Units (YPG) as an offshoot of the Kurdish PKK, which has fought an insurgency inside Turkey for the past 35 years.
To keep any Kurdish expansion in check, Turkey has launched two campaigns in Syria since 2016.
Last year it seized the key Kurdish enclave of Afrin after months of deadly bombardment.
On Tuesday, Turkey's President Recep Tayyip Erdogan said his country had "the right to eliminate all threats against its national security".
"God willing, we will carry the process (already started) to the next stage very soon," he said.
On Sunday, he expressed frustration with stalling US-Turkish talks in Ankara that aim to prevent a Turkish offensive by setting up a buffer zone along its southern border.
Turkey would like such a "security zone" to run 30 kilometres (18 miles) wide inside Syria and to be controlled by Turkish troops.
What do the Kurds want?
Across the frontier, the Kurds initially rejected any Turkish presence in a buffer zone inside Syria, demanding international monitors instead.
But on Monday, a top political official in the Kurdish region said they were prepared to be flexible.
Aldar Khalil told AFP they had agreed to a buffer zone of around five kilometres wide, but Turkey rejected the proposal.
"It wants to control the area all on its own," he said.
He also said the Kurds had been asking to take part in "safe zone" talks between Washington and Ankara, but that the latter had refused.
Syrian Kurdish forces have played a key role in the US-backed fight against the Islamic State group.
YPG-led forces ousted the extremists from the last scrap of their "caliphate" in March.
Today, they hold thousands of suspected jihadists in jail, as well as alleged IS family members in overcrowded camps.
A Turkish offensive "would make guarding the jails and camps difficult," Khalil warned.
Urgently needing support, the Kurds have opened channels with the Russia-backed regime of President Bashar Al-Assad.
"The conversation is ongoing" with the Kurds hoping to "negotiate a certain formula for these areas", Khalil said.
"But Damascus has not yet decided anything or made its true position clear despite the urgency of the situation," he added.
If diplomacy failed, he said, Kurdish forces would have no choice but to "resist" the Turks.
Can the US save the day?
The United States -- both a NATO ally of Turkey and backer of the Kurds -- is stuck in the middle.
Despite Kurdish fears, on Monday the Pentagon insisted any unilateral offensive by Turkey would be "unacceptable".
But finding an arrangement that will suit both sides is a tricky task, analysts say.
Nicholas Danforth, a fellow at the German Marshall Fund, said the Americans and the Turks had different goals.
"US safe zone proposals have focused on the narrow goal of keeping Turkey's border safe," he said.
But "Turkey envisions something like Afrin", which it now totally controls with its Syrian rebel proxies.
Ankara's latest threats come amid signs of growing hostility towards the Syrian refugee population in Turkey, the largest in the world at 3.6 million.
Fellow analyst Nicholas Heras said Erdogan also aims to "relieve domestic political pressure on himself by resettling tens of thousands of Syrian refugees back inside Syria".
Faced with Ankara's demands, the United States would likely seek a compromise in joint patrols, including both Turkey and the anti-IS coalition, said Heras, from the Center for a New American Security.
"If they give Erdogan the 'win' of forcing a Turkish military presence in some areas... in chaperoned patrols with the US and perhaps French forces, then the threat of further Turkish military action can be averted," he said.
But Washington is not asking the YPG-led Syrian Democratic Forces if they will accept a Turkish military presence, he added.
"It is telling the SDF this is a fait accompli."