Major powers, including arch-rivals Iran and Saudi Arabia who back opposing sides in Syria, meet for the first time Friday in search of a political solution to the devastating war.

Top diplomats from more than a dozen nations have gathered in Vienna to narrow differences over the fate of President Bashar al-Assad, who has defied Western demands to step down.

In a sign of its growing diplomatic clout, Iran, a longtime sponsor of Assad's regime, is joining the talks for the first time, months after striking a landmark nuclear deal with world powers.

"This meeting is very timely because it is the first time since the start of the Syrian conflict that all the major players are gathering," French Foreign Minister Laurent Fabius said as he arrived for the talks.

"We must fight more effectively against the terrorists" of the Islamic State (IS) group and Al-Qaeda affiliate, the Al-Nusra Front, he said.

"Then it is necessary to organise the political transition," Fabius added. "Mr Assad, who is responsible for a large part of the Syrian tragedy, cannot be considered the future of Syria."

German Foreign Minister Frank-Walter Steinmeier called the talks a "sign of hope for Syria and the region" and an "important first step" toward finding a solution to the conflict.

There was no talk of representatives of the Syrian regime or the opposition attending the discussions aimed at ending a four-year war that has claimed a quarter of a million lives.

In Washington, which along with its Arab and Turkish allies backs Syrian rebels, officials had expressed a cautious hope that the players would agree the outline of a transition that would eventually see Assad step aside.

- Iranian debut -

Just ahead of the talks, 40 people were killed Friday when rockets fired by Syrian government forces crashed into a market in a rebel-held area outside Damascus, the Syrian Observatory for Human Rights said.

US Secretary of State John Kerry, while warning against expectations of an immediate solution, has described the talks as "the most promising opportunity for a political opening we have seen".

"Now it is the right time to bring Iran to the table," Kerry said.

Russia, which has waged a month of intense air strikes against Assad's armed opponents, has also urged preparations for parliamentary and presidential elections in Syria.

But the idea has been rejected by rebels who say a vote would be impossible in the current circumstances, with millions of Syrians displaced, cities standing in ruins and two-thirds of the country in the hands of jihadists and other armed groups.

There are also doubts about whether Tehran and Moscow are ready to push Assad to step aside, particularly at a time when the Islamic State group is seeking to tighten its grip on swathes of the country for its self-proclaimed caliphate.

Even so, mounting international concerns about the outpouring of Syrian refugees and the growing jihadist threat could set the scene for some kind of political compromise, experts say.

"Overall, we have for the very first time around the table all the major actors and we have a situation of fatigue on the ground, so it could lead to a potential breakthrough," said Karim Bitar, of the Paris-based Institute for International and Strategic Relations.

- 'Removed by force' -

Even getting Iran and Saudi Arabia -- the Middle East's foremost Shiite and Sunni powers which back opposing sides in conflicts across the Arab world -- to sit at the same table would mark progress.

Saudi Foreign Minister Adel al-Jubeir, in an interview with the BBC, made it clear that Riyadh is sticking by its view that Assad must leave office quickly.

"He will go either through a political process or he will be removed by force," he warned.

Russia and Iran have made it equally clear that Assad has a right to play a role in an eventual transition towards a mooted unity government and later elections.

"Apparently those who have tried to solve the Syrian crisis reached the conclusion that without Iran's presence there will be no possibility of reaching a rational solution," Iran's Foreign Minister Mohammad Javad Zarif said, according to state news agency IRNA.

Iran and Lebanon's Shiite armed movement Hezbollah have troops in Syria to advise or support Assad's forces.

The West has accused Russia of concentrating its air campaign in Syria on moderate opposition groups opposed to Assad's rule, although Moscow says it is focusing on defeating IS and other "terrorist" organisations.

Russian deputy foreign minister Sergei Ryabkov said Moscow and Washington "fundamentally differ" over the Syrian issue.

"The use of force in any form in Syria without agreement of the legitimate government of this country would be unacceptable to us," he told the Interfax news agency, speaking in Russia.

The United States and its allies have conducted more than a year of air strikes against IS targets in Syria but have refused to coordinate their campaign with the regime.

Representatives from Britain, Egypt, France, Germany, Italy, Lebanon, the European Union and possibly other Arab states, were expected to attend Friday's talks.

China sent its vice foreign minister Li Baodong, saying there was "strong momentum for a political solution".

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