Iran, Turkey and Russian Federation have held five rounds of peace talks between the Syrian regime and rebel groups in Astana, Kazakhstan since January which supplement the UN-brokered Geneva talks that are meant to find a political resolution to the nearly seven-year conflict.
For his part, Syria's special envoy for Syria Staffan de Mistura called to "bring the momentum of Astana" to broader talks on finding a political solution to the Syrian conflict.
The de-escalations zones will all be in force for six months and extended if necessary based on the guarantor's consensus.
It reaffirmed that Syria will not waive its right to preserve its independence and territorial integrity and it will not halt its war to eradicate terrorism wherever it exists and whoever supports it.
At least two million people live in Idlib, the largest populated area held by rebels - including some nationalist Free Syrian Army factions that have taken part in the Astana talks.
An observer group says intense clashes are ongoing in the Syrian central province of Hama between pro-government forces and members of the Islamic State group.
The U.S. State Department has said that Washington "remains concerned with Iran's involvement as a so-called "guarantor" of the Astana process".
The guarantor-states of the Syrian ceasefire regime on September 15 finally agreed on the creation of a safe zone in Idlib, an issue that had been on the agenda for a long time as the region is mostly controlled by terrorists and rebels.
However, the opposition has failed to secure its chief demand: the ousting of Syrian president Bashar al-Assad. They expressed gratitude to President of Kazakhstan Nursultan Nazarbayev and Kazakhstani authorities for hosting the 6th International High-Level Meeting on Syria in Astana.
The parties at the Syrian peace talks in Kazakhstan have agreed the boundaries of the final de-escalation zone, Turkish and Russian diplomatic officials said Friday.
In addition to the cessation of fighting, the de-escalation deals also include ending flights and air strikes by the regime or US -led anti-Daesh coalition.
"We have welcomed the Astana talks and we've recognized the importance of the creation of de-escalation zones and we hope that mechanisms in order to make them fully effective will be able to be put in place", Guterres said.
Three de-escalation zones previously agreed on saw a dramatic drop in hostilities.
The participants agreed to a fresh round of talks in Astana in October.
De-escalation zones, proposed by Russian Federation, are aimed at separating extremist groups, including self-styled Islamic State terrorists and Jabhat al-Nusra from the moderate opposition.
Those issues are still mainly being handled by the United Nations at parallel talks in Geneva, but progress there has been negligible.