No group has claimed the four activists' abduction and they have not been heard from since.
Ankara-backed rebels played a key role in supporting Sharaa's Islamist group Hayat Tahrir al-Sham (HTS), which headed a rebel alliance that seized Damascus on December 8, toppling longtime ruler Bashar al-Assad.
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.
Assad fled Syria following a lightning offensive spearheaded by the Islamist Hayat Tahrir al-Sham (HTS), more than 13 years after his crackdown on democracy protests precipitated one of the deadliest wars of the century.
Darwish was detained for months by one of the most feared branches of the former government's many-tentacled intelligence services.
Syrians will not miss Assad, a brutal ruler who failed his people.
The offensive, which took just 10 days to sweep across Syria and take the capital Damascus, stunned the world and brought an end to more than a half a century of brutal rule by the Assad clan.
"We would like to see the situation in the country stabilised somehow as soon as possible," Kremlin spokesman Dmitry Peskov told reporters.
"Their human capital, their experience will allow the country to flourish," Bashir said in an interview published Wednesday.
Syria's civil war has prompted the first withdrawal of crop seeds from a "doomsday" vault built in an Arctic mountainside to safeguard global food supplies.
The United States will increase the number of refugees it takes in by 15,000 over each of the next two years, bringing the total to 100,000 by 2017, US Secretary of State John Kerry says.
Austria saw the arrival of at least 10,000 migrants, amid bitter rows among EU nations on how to handle the growing crisis.
Migrants stranded in Croatia are making renewed efforts to head north despite moves by Slovenia and Hungary to hold them back.
Croatia closes seven of its eight road border crossings with Serbia following a huge influx of migrants.
The flow of migrants through the Balkans picks up again with thousands entering Croatia, and the number doubling in a day for Germany.
The Australian government has confirmed it has launched its first air strikes inside Syria against targets of the so-called Islamic State (IS).
Hungarian riot police fire tear gas and water cannon to force back migrants who broke through gates at the now closed Serbia-Hungary border.
Hundreds of migrants are stranded at the Serbia-Hungary border after the Hungarian government closed the frontier with a new razor-wire fence.
Russia's recent movements near Syria's city of Latakia suggest that Moscow plans to establish a "forward air operating base" there, the US has said.
