Kurdish forces in north-eastern Syria are preparing to remove heavy weapons and fighters from the strip along the Turkish border that is to be protected by a safe zone agreed to by the US and Turkey.
The People's Protection Units (YPG) militia withdrew from the Tal Abyad and Ras Al Ain border positions in recent days, proving it is serious about talks, the Kurdish-led authority in north and east Syria said.
The withdrawal is a sign of progress in talks between Washington and Ankara to resolve differences over the presence along the border of Kurdish fighters, who are US allies that Turkey regards as enemies.
After Ankara repeatedly warned it would launch a military incursion into north-east Syria against the YPG, Turkey and the US this month said they had agreed on the first stages of a security deal along the border.
The two countries gave no details of the deal on what Turkey has called a "safe zone" inside Syria.
It came after months of stalemate over how far the zone should extend into north-east Syria, which is still a main source of friction, and who should command forces patrolling it.
Senior Turkish and Syrian Kurdish officials said points of contention remained over the zone, which Turkey wants to extend 32 kilometres into Syria.
The YPG leads the US-backed Syrian Democratic Forces alliance, which controls much of north and east Syria.
US troops have been stationed in the region for years, training and arming the fighters who seized territory from ISIS.
"This plan maintains security in north-east Syria so ISIS cannot re-emerge, and allows the coalition and our partners to remain focused on achieving the enduring defeat of ISIS," said Commander Sean Robertson, a Pentagon spokesman.
US President Donald Trump said late last year that he would withdraw American troops, sparking fears among the Kurds of a Turkish attack, but he has so far held back from doing so.
US support for the YPG has enraged Turkey, which considers the Kurdish fighters a security threat linked to insurgents at home.
SDF spokesman Mustafa Bali said the strip along the border would vary between 5km and 14km, and include rural areas or military positions, not cities or towns.
The YPG and SDF will dismantle barricades there and hand over control to military councils of local fighters, Mr Bali said.
He said Turkish and US-led coalition forces would patrol the border strip but be based inside Turkey.
The deal creates "a security mechanism, not a safe zone, that assuages Turkey's claims of fearing for its national security", Mr Bali said.
Senior Kurdish official Badran Jia Kurd said there was no "final deal yet as the talks are ongoing and the threat remains".
Mr Kurd said the YPG move showed commitment to the agreements.
Ankara has already sent its military twice into northern Syria to drive the Kurds from its borders in recent years.
A source said that although Washington and Ankara were still discussing the extent of the zone, they agreed to start work on one stretch of the border.
"The safety mechanism arrangement is being implemented in phases," the source said.
The source said US-Turkish joint patrols would monitor the removal of heavy weapons, fortifications and tunnels, along with the YPG presence between Tel Abyad and Ras Al Ain, two Syrian border towns about 100km apart.
That stretch is about a quarter of the whole border that would be covered by the zone.
A senior Turkish official said Ankara and Washington had bridged some, but not all, of their differences.
"There was a rapprochement but our insistence on the 20 miles persists," the official said.
"The United States has taken steps to improve this but they are still not enough. It is not possible for us to accept the SDF's presence there."
President Recep Tayyip Erdogan said on Monday that Turkish troops would enter the planned zone "very soon", after setting up a joint military centre with the US to oversee the operation at the weekend.
Turkish drones and helicopters had already flown over the region, Mr Erdogan said.
He said Turkey had prepared to carry out its own plans if its expectations were not met.
As long as "provocations" continue from across the border, "it is not possible for us to wait with our hands tied", Mr Erdogan told Russian President Vladimir Putin on Tuesday.