Negotiators in Vienna set new deadline for talks between parties to the conflict, with the aim of establishing a ceasefire by May

The Isis attacks in Paris have galvanised international efforts to end the war in Syria, with a new deadline set for negotiations between the warring parties and for a country-wide ceasefire.

There is still no sign of agreement, however, on the key question of the future of the Syrian president, Bashar al-Assad.

The grim news from France inevitably overshadowed talks in Vienna on Saturday, but the widest forum yet convened to discuss the crisis exceeded expectations. Russia, which launched airstrikes in Syria in late September, played a bigger role than it has done until now.

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The UN secretary general, Ban Ki-moon, called it a “rare moment of diplomatic opportunity”. Barack Obama and other leaders meeting at the G20 summit in Antalya on Sunday, repeated demands for greater international cooperation.

The US, Russia, Britain, France, Iran and Saudi Arabia signed a statement supporting a 1 January deadline for the start of talks between the Syrian government and opposition, with the aim of agreeing a ceasefire by 14 May.

The 19 participants in the international Syrian support group (ISSG) also mandated Jordan to draw up a list of anti-Assad rebel groups which could take part in peace talks. That will exclude Isis and Jabhat al-Nusra, al-Qaida’s Syrian affiliate, which are both banned by the UN. Both could be attacked even when a broader ceasefire is in place, the statement said.

The UN security council, which has been bitterly divided over Syria since the crisis began in March 2011, would also mandate a mission to monitor a ceasefire and political transition.

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Forty Syrian rebel groups issued a joint statement condemning the Paris atrocities as the work of “the Assad regime and its creation Isis.” They did not include one of the most important Islamist groups, Ahrar al-Sham, the Free Men of Syria, whose foreign relations spokesman issued his own statement. “Terrorism is an indivisible whole,” Labib Nahas tweeted. “Assad cannot create Daesh [Isis] in Syria and then help in the global war against terror.”

Assad blamed the Paris atrocities on what he called France’s mistaken policies when he met French MPs in Damascus on Saturday. “The question that is being asked throughout France today is, was France’s policy over the past five years the right one?” he said. “The answer is no.” France has made clear it will continue to attack Isis and call for Assad’s departure.

Syrian opposition forces fear western countries will come to accept Assad’s argument that they should cooperate with him to fight the jihadi as the lesser evil. Assad’s position is that all those who have fought his government are terrorists. Russia and Iran, his staunchest allies, share his view, but Saudi Arabia, a key backer of Islamist rebels - though not of Isis – disagrees.

Under the Vienna agreement, the sponsoring countries of each group covered by the ceasefire would be responsible for making sure that group upholds it. “All members … pledged as individual countries and supporters of various belligerents to take all possible steps to require adherence to the ceasefire by these groups or individuals they support, supply or influence.” the UN said.

The requirement carries obvious difficulties in terms of definition and transparency.

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Negotiations between the parties to the Syrian conflict were to establish a “credible, inclusive and non-sectarian” transitional government in Damascus that would set a schedule for drafting a new constitution and holding a free and fair UN-supervised election within 18 months, according to the UN statement.

It was clear, however, that Russia and the US have again had to agree to disagree about Assad. The Paris attacks “show that it doesn’t matter if you’re for Assad or against him,” said the Russian foreign minister, Sergei Lavrov. “Isis is your enemy.”

The UN special envoy for Syria, Staffan de Mistura, called the 18-month deadline to establish a new Syrian government “very challenging but possible”.

The US, Britain and France say they want to accelerate a twin-track approach to Syria, intensifying international diplomacy and taking military action against Isis. Last week’s capture of the Iraqi town of Sinjar by US-backed Kurdish forces is being held up as a model for future operations.

Isis, in their view, is a symptom of political failings in both Iraq and Syria. The Vienna participants are to meet in Paris before the end of the year to review progress toward a ceasefire and the selection of delegations for the Syrian talks.