Russia's Foreign Minister Sergei Lavrov (R) and US Secretary of State John Kerry address the media before a conference on the Syria conflict in Vienna, Austria. AFP

AFP/Beirut

Syrian opposition members on Sunday said an ambitious plan by world powers for a political transition in their war-ravaged country was unrealistic, though some reacted with cautious optimism.

Global diplomats gathered in Vienna agreed Saturday on a fixed calendar for Syria that would see a transition government in six months and elections within 18 months.

A final statement after the meeting said the goal was to bring Syrian government and opposition representatives together by January 1.

Anas al-Abdeh, a member of the opposition National Coalition, said the statement remained "unclear" but marked some progress. 

"A ceasefire is in principle a good thing as it will alleviate people's suffering. But the most important thing is observing its implementation," he told AFP by telephone. 

But Samir Nashar, a fellow Coalition member, derided the Vienna plan as "frustrating and unrealistic", insisting it "will not lead to a political solution".

"How do they expect that after everything that has happened in Syria, people will just kiss each other in the streets?" he said.

With millions displaced and the country's infrastructure devastated, Nashar said Syria would need much longer to hold free and fair elections.

He warned that the omission from the Vienna statement of a key aim of the uprising -- the departure of President Bashar al-Assad -- would strengthen claims by jihadist groups that the international community is "conspiring against the Syrian people". 

"This was the biggest blow... This is a gift to Daesh, to extremism," he said, using the Arabic acronym for the Islamic State jihadist group. 

 'Paving the way' for peace

 On Sunday, a top Iranian official said several countries involved in the peace talks had tried to bar Assad from future elections, but Tehran, a key backer of Syria's regime, insisted the demand be withdrawn.

Assad's departure has been the centrepiece of opposition demands since Syria's uprising began in 2011 and Western powers -- including the United States, France and Britain -- have called for him to step down.

Saturday's talks in Vienna were the second round of the broadened diplomatic efforts to reach an end to Syria's war, which has left more than 250,000 people dead. 

Top diplomats from 17 countries as well as three international organisations met the day after a deadly coordinated assault claimed by IS in Paris left at least 129 people dead. 

Hassan Abdel Azim, the head of the National Coordination Committee for Democratic Change, an internal opposition group, said Vienna had set out a "practical plan".  

"Everything that happens in Vienna, we agree with and is in line with the vision of the NCCDC for a political solution," he said.

But he cautioned that the process remained "open to amendments".

Confidence-building measures, including a ceasefire, prisoner releases, and aid deliveries were all "steps that pave the way for a transitional period, forming a government, and holding elections," Abdel Azim said. 

Ceasefire 'far from reality'

Though Syria's conflict began with anti-government protests, it has since fractured into a multi-front war between the regime, rebels, Kurds, and jihadists, with shifting military alliances throughout the country. 

UN peace envoy to Syria Staffan de Mistura was tasked, as part of the Vienna talks, with bringing about a ceasefire between warring groups. 

But Fares Buyush, head of the Fursan al-Haq rebel brigade fighting in north and northwestern Syria, said the decisions reached in Vienna were "far from reality" and that a ceasefire would be "very difficult" to implement. 

And Asaad Hanna, a spokesman for the Division 101 rebel group, said Vienna's goals -- including a ceasefire -- were divorced from the reality of forces fighting both IS and Assad's troops. 

"Just because the international community holds a press conference, it doesn't mean they're going to stop Daesh," Hanna said by phone. 

"The uprising is happening on the ground. How can the meetings exclude the decision-makers on the ground, who would be the ones implementing the ceasefire?"