US Secretary of State John Kerry waves as he leaves after a meeting with top diplomats in Vienna on October 23, 2015 in Vienna. (AFP)
Key players in the diplomatic push to end the Syrian war were to meet Today seeking common ground over a political transition, ahead of Iran's first-time appearance at international talks on the conflict.
Top diplomats from Russia, the United States, Saudi Arabia and Turkey will hold the second round of talks on Syria in less than a week, ahead of the larger meeting of regional players, including Iran, on Friday.
Iran's Foreign Minister Javad Zarif will join representatives from Britain, Egypt, France, Germany, Italy, Lebanon and the European Union as the international community seeks to end more than four years of bloodshed.
It will be the first time all the major players in the conflict are in the same room, though there has been no mention of either the Syrian government or the opposition attending.
The inclusion of Iran a key ally of Syrian President Bashar al-Assad in this week's meetings marks a crucial shift after Tehran was excluded from earlier talks, mainly because of opposition from Washington and Riyadh.
US Secretary of State John Kerry cautioned that this week's talks would not secure an immediate political solution, but nevertheless represented the best hope available.
"While finding a way forward on Syria will not be easy it's not going to be automatic it is the most promising opportunity for a political opening we have seen," Kerry said just before he set off for Vienna.
"The challenge that we face in Syria today is nothing less than to chart a course out of hell," he added.
Britain's Foreign Office said the United Arab Emirates, Qatar, and Jordan will also attend Friday.
Washington is at loggerheads with Moscow over Syria, accusing Russian forces of concentrating their air campaign there on moderate opposition groups opposed to Assad's rule.
Moscow says its bombing campaign is targeting Islamic State jihadists and also other "terrorist" groups.
But Kerry stressed that the US and Russia also shared "common ground", arguing that both want "a united, secular Syria" in which citizens can choose their own leader through elections.
Divisions over Assad's futureAfter months of failure, efforts to find a breakthrough have gained pace as hundreds of thousands of Syrians have fled their shattered country.
Serious divisions remain over when or whether Assad should step down and four-way Russia-US-Saudi-Turkey talks in Vienna last Friday failed to make a breakthrough.
On one side, Russia and Iran are backing Assad's forces on the ground and say Damascus must be helped to defeat "terrorism" before a political process can take shape.
On the other, the United States and its key regional allies Turkey and Saudi Arabia are supporting groups fighting Assad and insist he must go.
French Foreign Minister Laurent Fabius said Wednesday that France and its allies had agreed on the need for a "precise timetable" for Assad's departure.
Meanwhile, the Pentagon announced plans to step up attacks on Islamic State jihadists in Syria and Iraq, with Defence Secretary Ashton Carter saying he expected more airstrikes and even possible "direct action on the ground".
The dynamic in the Syrian conflict shifted after Russia launched its air campaign on September 30, claiming it was targeting IS fighters.
Russian warplanes struck 118 "terrorist" targets in Syria over the
past 24 hours, the highest daily total yet, Moscow's defence ministry said Wednesday, attributing the rise to fresh intelligence.
Iran is believed to have sent thousands of troops and Hezbollah militia fighters to support Assad's forces.
But the US believes they will struggle to defend Assad for long, which is why they have been forced to engage diplomatically.
More than 250,000 people have been killed in Syria's brutal conflict since it began in March 2011 following a bloody crackdown on protests against Assad's rule.