VIENNA: Some 20 countries and international bodies meet in Vienna again Saturday groping for a way out of Syria's horrific civil war, with deep divisions over President Bashar al-Assad's future and which rebel groups to back.

The second such gathering in two weeks comes as Islamist rebels suffer a number of setbacks in Syria and Iraq at the hands of Assad's army helped by Russian air strikes, and Kurdish forces backed by the US.

Before leaving for Vienna, US Secretary of State John Kerry warned that a quick breakthrough was unlikely in the talks which bring together key players like Russia, Iran, Saudi Arabia and UN special envoy Steffan de Mistura.

“I cannot say... that we are on the threshold of a comprehensive agreement, no,” said Kerry, who arrived in the Austrian capital on Friday afternoon for preliminary talks with his Saudi, Turkish and UN counterparts.

“The walls of mistrust within Syria, within the region, within the international community are thick and they are high.”

In over four 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, where authorities have been on alert after several deadly jihadist attacks this year.

In the latest, 120 people were killed in a wave of coordinated attacks in Paris on Friday, including a massacre during a rock concert by men shouting “Allahu akbar” and blaming France's military intervention in Syria.

At the last 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'

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.

The attack, the biggest ever attributed to IS in Lebanon, harked back to a campaign against the Shia movement Hezbollah between 2013 and 2014, ostensibly in revenge for its military support to Assad.

But in Syria, Assad's army scored its second important victory in two days on Thursday by capturing Al-Hader, a former opposition bastion largely controlled by Al-Qaeda affiliate Al-Nusra Front and other Islamists.

Bolstered by the Russian air strikes, the breakthrough came just 48 hours after regime forces broke a siege by IS of the Kweyris air base in the east of Aleppo province.

On Friday, the US-backed Syrian Democratic Forces coalition of Syrian Arab and Kurdish fighters it had ejected IS from Al-Hol, a key position on the border with Iraq for the supply of arms and equipment.

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 that it is “reasonably certain” that it killed “Jihadi John”, the notorious militant with a British accent in grisly IS execution videos, in a drone strike in Syria.

US President Barack Obama said the US has now halted the expansion of IS, calling in an interview broadcast Friday for a stepped up drive to “completely decapitate” the militants' operations.