Turkey's intelligence chief has held multiple meetings with his Syrian counterpart in Damascus in the last few weeks, a sign of Russian efforts to foster a thaw between states on opposite sides of the Syrian war, four sources said.
Turkey is one of several foreign states with troops in Syria. Here's a summary of the main armies on the ground and how they got there:
IRAN AND ALLIES
Iran deployed its elite Revolutionary Guards as early as 2012 to help its ally, President Bashar al-Assad, counter rebels fighting to topple him.
Tehran has always described Iranian forces as playing an advisory role at the Damascus government's invitation. But hundreds of Iranians have been killed.
Alongside Iranians, Shi'ite Islamist groups backed by Tehran have played a vital combat role. Spearheaded by Lebanon's Hezbollah, they have included groups from Afghanistan and Iraq.
Iranian-backed forces have a presence across much of government-held Syria, including at the Iraqi border.
Israel has mounted air strikes targeting Iranian and Iran-backed forces.
THE UNITED STATES
The US military intervention in Syria began in 2014 with air strikes against the Islamic State jihadist group that had declared its rule over a third of Syria and Iraq.
An initially small contingent of US special forces deployed to Syria, working with a Kurdish-led force, the Syrian Democratic Forces (SDF), fighting to drive Islamic State from areas it had captured in Syria's north and east.
Declaring the battle with Islamic State almost won, President Donald Trump announced in 2018 he wanted to pull out US troops.
But the plan was softened within a year after facing criticism for leaving a void that Iran and Russia would fill.
US forces remain in Syria and continue to support the SDF.
US troops are also stationed at Syria's Tanf garrison near the intersection of the borders of Jordan and Iraq, where they support a Syrian rebel force.
Assad's government views the US forces as occupiers.
Turkey has mounted four operations in Syria since 2016, and has forces on the ground in the north where it backs rebels.
Its first incursion targeted both Islamic State and the Syrian Kurdish YPG - part of the SDF - which is seen by Ankara as a security threat because of links to the Kurdistan Workers' Party, which has been waging an insurgency in Turkey since 1984.
Turkey's presence expanded in 2017 when it struck a deal with Russia and Iran that resulted in Turkish forces deploying at 12 positions in the northwestern Idlib region, an area largely controlled by anti-Assad jihadists.
This was followed in 2018 by an offensive targeting SDF-controlled Afrin, and another incursion in 2019 into SDF territory between the border towns of Ras al Ain and Tel Abyad.
The following year Turkey poured thousands of troops into the Idlib region to stem an offensive by Russia-backed Syrian government forces targeting rebels supported by Ankara.
Damascus views Turkey as an occupying power.
Russia deployed warplanes in Syria in 2015, establishing an air base in Latakia province from where it carried out an air campaign that tilted the conflict Assad's way.
Coordinated with Iran, the deployment expanded a Russian military presence dating to the Cold War, when the Soviet Union established a naval base at the Syrian Mediterranean port of Tartus.
While their main role has been air power, Russian forces also have a presence on the ground in government-held areas, with Russian military police deploying during attempts to de-escalate fighting.