Multiple "terror" attacks in Paris increased the pressure on some 20 countries and organisations meeting in Vienna on Saturday to overcome deep divisions and help end Syria's horrific civil war.
Witnesses said that the gunmen who killed at least 120 people in Friday's wave of attacks shouted "Allahu akbar" ("God is greatest") and blamed France's military intervention in Syria against Islamic State (IS) extremists.
The Vienna talks, involving key players Saudi Arabia and Iran as well as Russia, come however as rebels suffer a number of setbacks in Syria and Iraq, with Russian strikes helping President Bashar al-Assad's regime regain territory.
Arriving in the Austrian capital, Saudi Arabia's Foreign Minister Adel al-Jubeir said that the "heinous" attacks in Paris are in "violation and contravention of all ethics, morals and religions".
"The kingdom of Saudi Arabia has long called for more intensified international efforts to combat the scourge of terrorism in all its forms and shapes," he said.
US Secretary of State John Kerry, who on Friday met with Jubeir as well as the UN special envoy for Syria, Steffan de Mistura, had warned before heading to the talks that a quick breakthrough was unlikely.
"I cannot say... that we are on the threshold of a comprehensive agreement, no," Kerry had said on Thursday.
"The walls of mistrust within Syria, within the region, within the international community are thick and they are high."
In almost five years, fighting between Assad's regime and rebel groups as well as Islamic State (IS) militants has killed over 250,000 people and forced millions into exile, leaving many of them stranded in neighbouring states.
Others have headed to Europe, causing major splits in the European Union over how to stem the flow and share out the new arrivals among the bloc, boosting populist parties across the continent.
At the last Syria talks on October 30, the participants urged the United Nations to broker a peace deal between the regime and opposition to clear the way for a new constitution and UN-supervised elections.
Building on that, this round of talks in the Austrian capital will try to agree on a roadmap for peace that would include a ceasefire between Assad's forces and some opposition groups.
- 'Assad must go' -
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But a key issue -- which was absent from the last meeting's declaration -- remains Assad's future.
Western and Arab countries want him out of the way in order to allow a transitional government to unite the country behind a reconciliation process and to defeat IS.
British Foreign Secretary Philip Hammond said on Friday that Assad "has to go".
He added, however, that Western powers "recognise that if there will be a transition he may play a part, up to a point, in that transition".
But Russia, carrying out air strikes against Syrian rebels since late September, is together with Iran sticking with Assad, seeing him as the best bulwark against IS.
"Syria is a sovereign country, Bashar al-Assad is a president elected by the people," Russian President Vladimir Putin said in an interview released Friday.
That aside, the talks will focus on deciding which of the Syrian government, rebel and opposition factions -- none of whom will be represented at the talks -- will shape the country's future.
But deciding which of the many opposition groups are moderate enough to be acceptable and which to sideline as "terrorists" is likely to be no easy task.
"It will require deep breaths on several sides, including the US side," Hammond said on Tuesday.
- Violence rages -
On the ground, widespread fighting was raging in Syria and Iraq and further afield, with IS claiming a twin bomb attack in Beirut on Thursday that killed 44 and wounded least 239.
Within Syria, Assad's army scored an important victory Thursday by capturing Al-Hader, a former opposition bastion largely controlled by Al-Qaeda affiliate Al-Nusra Front and other Islamists.
In Iraq, Kurdish peshmerga forces and Yazidi minority fighters, backed by US-led air strikes, liberated the town of Sinjar and cut a key IS supply line, Iraqi Kurdish leader Massud Barzani said Friday.
And the US military said Friday it was "reasonably certain" that it killed "Jihadi John", the notorious militant with a British accent seen in grisly IS execution videos, in a drone strike in Syria.