The meeting in Vienna will seek to close the gap between the US and its allies, who support the rebels, and the key foreign allies of the Syrian government, Russia and Iran.
It is the first time Iran has been involved in the diplomacy.
Meanwhile, activists in Syria say a bombardment by government forces killed at least 40 people near Damascus
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Footage uploaded to the internet - which cannot be independently verified - showed bodies strewn across the ruins of a marketplace in the rebel-held suburb of Douma.
Douma is often targeted by the government, and rebels fire rockets from there into Damascus.
The four-year-old war in Syria, which began with an uprising against Mr Assad, has left 250,000 people dead and forced half the country's population - or 11 million people - from their homes.
Russia and Iran have recently stepped up their military involvement in the conflict, backing forces loyal to Syrian President Bashar al-Assad
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The US, Turkey, Saudi Arabia and other Gulf Arab nations have long insisted that he cannot play a long-term role in Syria's future
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Iran's growing role
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"He will go either through a political process or he will be removed by force," Saudi Foreign Minister Adel al-Jubeir told the BBC ahead of the talks.
Meanwhile, Iranian Foreign Minister Mohammad Javad Zarif said that other powers had realised that there was no way reaching "a reasonable solution" to the Syrian conflict without involving Tehran.
PR
