The first meeting between Syrian government and opposition negotiators over the war-torn country's future constitution has been hailed as a "landmark" by UN Secretary General Antonio Guterres.

The meeting on Wednesday saw the two sides sit face-to-face in Geneva for the launch of the constitutional review committee's work. 

At a conference on Thursday focusing on mediation in Istanbul which was organized by the Turkish government, Guterres said "Yesterday's first meeting of the Constitutional Committee was a landmark, a foundation for progress. 

"I hope this will be the first step towards a political solution that will end this tragic chapter in the lives of the Syrian people, also to create the opportunity for all Syrians to return to their places of origin, in safety and dignity, to end their status as refugees."

The United Nations-brokered committee is comprised of 150 delegates which are divided equally among President Bashar al-Assad's government, the opposition and civil society. 

The conflict has claimed 370,000 lives over the past eight years. 

Experts say that Assad and his forces have made major gains against the opposition, and have little to lose at the talk and will walk away rather than making any significant compromises. 

Guterres said that he was "very concerned about the situation in Idlib". The region comprises three million residents, half of whom  are from other parts of the country. It is Syria's last major rebel bastion. 

Assad said last week that winning the Idlib front would be central to winning the war and that his troops had completed preparations for such an offensive.

Guterres said that he was repeating his "call for maximum restraint, de-escalation and the protection of civilians and civilian infrastructure on all fronts in Syria."