Russian forces have begun to deploy towards the Turkey-Syria border, as part of deal to remove Kurdish troops.
Units were seen crossing the Euphrates heading towards the city of Kobane.
Under the deal agreed by Russia and Turkey, Russian and Syrian forces will remove Kurdish fighters from a large stretch of the border.
Turkish troops will continue to control an area they took during a recent offensive against the Kurdish fighters, whom Turkey regards as terrorists.
Part of Turkey's plan is also to create a "safe zone" along the border that will house some two million of the Syrian refugees it hosts.
The Turkish offensive began after the US announced a sudden and unexpected withdrawal of its troops from northern Syria. The US troops had been supporting the Kurdish fighters, who have been allies in the fight against the Islamic State (IS) group in the region.
What's the latest on the ground?
Russian forces crossed the Euphrates at noon (09:00 GMT) and "advanced towards the Syrian-Turkish border", Russian media said, quoting the nation's defence ministry.
A convoy of military police has arrived in Kobane, the ministry said.
The timing fits with what was agreed in the deal negotiated by Turkish President Recep Tayyip Erdogan and Russia's Vladimir Putin in the Black Sea resort of Sochi on Tuesday.
At noon, Russian and Syrian forces were to move in to oversee the Kurdish pullback in an area from the Euphrates, just east of the city of Manbij, right up to the Iraqi border in the east.
However, this would not include the area between the towns of Ras al-Ain and Tal Abyad which is now under control of the Turkish military. Under the deal, the Turks will retain control there. Turkey's military says the US has told it that all Kurdish fighters have now left that area.
It will also not include the area around Qamishli. The memorandum of understanding between Turkey and Russia makes the Kurdish-majority city an exception but gives no details as to why.
What else is in the deal?
Kurdish fighters have been given 150 hours from noon on Wednesday to pull back 30km (18 miles) along the border.
After the deadline expires on 29 October, Turkish and Russian troops will begin joint patrols in areas described as "in the west and the east of the area" of the Turkish offensive.
The statement from Russia and Turkey also says Kurdish forces "will be removed" from Manbij and the town of Tal Rifat, 50km to the west of Manbij – both of which lie outside the operation area.
On Wednesday, Russia's defence ministry said the Syrian government would establish 15 border posts with Turkey.
What has been the response to the deal?
Kurdish militias and political leaders have made no comment on whether they will agree to the demands.
Kremlin spokesman Dmitry Peskov said if the Kurds did not retreat, the Syrians and Russians would fall back and leave them to face "the weight of the Turkish army".
Syria's President Bashar al-Assad has in the past raised concern about foreign interference in Syria but the Kremlin said he had thanked President Putin and "expressed his full support" for the deal.
Iran's foreign ministry said the agreement was a positive step and that it backed any move to restore stability in the region.
Nato Secretary-General Jens Stoltenberg said on Wednesday it was too early to judge the deal, and called for a "real, negotiated, political solution in Syria".
The US ambassador to Nato, Kay Bailey Hutchison, said the US backed Germany's plan for an internationally controlled security zone in the area, although she said direct US involvement was unlikely.
In Geneva on Wednesday, a Syrian Kurdish man set himself on fire outside the HQ of the UN's refugee agency and is being treated for injuries.
How did we get here?
A US-led coalition relied on Kurdish-led forces to battle IS militants in northern Syria over the past four years, but they are dominated by the Kurdish People's Protection Units (YPG) militia, which Turkey sees as a terrorist organisation.
Just over two weeks ago President Donald Trump announced that he would be withdrawing US troops from Syria. Soon Read More – Source