Iran has been invited to attend the second round of Vienna peace talks, prior to the meeting, US Secretary of State John Kerry met with his Iranian counterpart Mohammad Javad Zarif on Thursday to talk about Syria's future.
The meeting is part of two days of diplomatic efforts, and will include top international players under the same roof for the first time.
Tehran and Moscow are both essential in their support for Assad's government, while Turkey, the United States, Saudi Arabia and their Western and Gulf allies are against the Syria regime.
Iran has not shown any signal that may indicate a change in stance over Assad. A senior Iranian official told Reuters that "there was no candidate to replace Assad, describing him as the only one who can prevent Syria from collapse," and added that it was necessary to assist Assad in his fight against militants, who have captured large parts of Syria and Iraq.
Bashar al Zoubi of the Yarmouk Army, a group affiliated to the Free Syrian Army told Reuters that "Iran is part of the problem and not the solution, and its participation in the meeting will prove that to the world," he later added. "This meeting was accepted by Saudi Arabia and Turkey to expose Iran."
Anti-Assad Syrian National Council (SNC) said that Iran's participation in the talks would undermine the political process. "Iran has only one project - to keep Assad in power... they don't believe in the principle of the talks," the coalition's Vice-President Hisham Marwa said on Thursday.
The war in Syria has, to date, claimed the lives of an estimated 250,000 people, most of whom were killed by regime air strikes and indiscriminate barrel bombings from helicopters on civilian areas.
Approximately half the country’s population has been displaced, with an estimated 6.7 million people seeking refuge elsewhere in Syria and 5 million moving into neighbouring countries such as Turkey, Egypt, Lebanon and Jordan.
This year alone, around half a million refugees, mostly Syrians, have entered Europe, hoping to gain asylum in economically well-off EU countries.