By BBC News
The US is intensifying diplomatic efforts to end the "hell" of Syria's civil war even as it increases support for moderate rebels, US Secretary of State John Kerry has said.
Mr Kerry is travelling to Vienna for talks with foreign ministers on ending the four-and-a-half year conflict. Iran is for the first time taking part in such talks, which will also include Russia, Saudi Arabia and Turkey.
Russia and Iran both support Syrian President Bashar al-Assad. The US, Turkey, Saudi Arabia and other Gulf Arab nations insist he cannot play any long-term role in the country's future.
Foreign ministers from the UK, France, Germany, Egypt, Lebanon and the EU have also confirmed they will attend the meeting, and other Middle Eastern powers are also expected.
"The challenge that we face in Syria today is nothing less than to chart a course out of hell," Mr Kerry said in a speech at the Carnegie Endowment for International Peace think-tank in Washington DC.
"While finding a way forward on Syria will not be easy... it is the most promising opportunity for a political opening we have seen."
He added: "At the end of the day, nothing would do more to bolster the fight against Daesh [the Islamic State militant group] than a political transition that sidelines Assad so that we can unite more of the country against extremism."
But in his speech, Mr Kerry stressed that the US and Russia also shared "common ground", arguing that both want "a united, secular Syria". Iran is believed to have spent billions of dollars over the past four years propping up President Assad's government, providing military advisers and subsidising weapons.
However, Syria's political opposition has warned that Iran's involvement will only complicate the meeting in Vienna. Both Iran and Russia have recently stepped up their military role in the Syrian conflict. Iran has long acknowledged sending military advisers to Syria, but has denied the presence of any ground forces.
Despite that, unconfirmed reports earlier this month said that hundreds of Iranian troops had arrived in Syria. They were reported to be joining government forces and fighters from the Lebanese Shia Islamist movement, Hezbollah, in assaults on rebel positions in northern and central Syria.
Russia began its military intervention in Syria at the end of last month, launching air strikes in support of President Assad. Washington has accused Moscow of concentrating its air campaign there on moderate opposition groups rather than on the Islamic State militant group.