Turkish army installs 'observation posts' in Syria's Idlib
Three zones are already in place - in Eastern Ghouta near Damascus, in central Homs, and in parts of southern Syria - which are being monitored by Russian military police.
The fourth de-escalation zone includes Idlib but also parts of the neighbouring Latakia, Hama and Aleppo regions.
In late September, Russian and Syrian regime airstrikes killed more than 150 civilians and injured dozens more in over a week of heavy bombings in the opposition-held Idlib province.
September was the deadliest month for Syrians this year, with at least 3,000 people killed following intense bombardments of Idlib and Islamic State group-occupied territories.
Turkey supports Syrian opposition fighter and has called for the removal of Syrian regime president Bashar al-Assad but has worked closely over the last few months with Russia, a key backer of the Assad regime.
The Syrian conflict began when the Baath regime - in power since 1963 and led by Assad - responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings.