US Secretary of State John Kerry arrived Thursday in Vienna ahead of talks that are aimed at ending the Syrian war and involve 20 countries, including bitter rivals and close allies of President Bashar al-Assad.
Kerry - who said that finding a solution in Syria is like charting a "course out of hell" but also that Friday's negotiations present "the most promising opportunity for a political opening" - will meet Russian Foreign Minister Sergei Lavrov later Thursday.
Iran, a major al-Assad backer, will be attending the international peace talks for the first time.
The one-day meeting is seen as the beginning of a dialogue among the regional rivals and other key players in the conflict, which started in 2011, but no breakthrough is expected.
Russia's Foreign Ministry on Thursday confirmed the participation of Iraq and Lebanon.
Among the other countries expected to attend are Saudi Arabia, Turkey, Jordan, Egypt, the United Arab Emirates, Qatar, Germany, Britain, France, Italy.
There will be no representatives from either the al-Assad government or the Syrian opposition, which slammed Iran's participation as "a dangerous development."
Iran provides vital military and financial support to al-Assad and was blocked by Washington from an unsuccessful UN-brokered peace conference last year for not accepting an internationally backed road map based on replacing al-Assad's government.
The United States has lent support to anti-al-Assad forces while Saudi Arabia and Turkey have been major backers of Syrian rebel groups.
An estimated 250,000 people have been killed in the conflict, which has also driven more than half the country's pre-war population of 22.4 million people from their homes, with almost 4.7 million seeking refuge abroad.
The wave of refugees has strained the capacities of Syria's neighbours and thrown European migration policy into disarray as hundreds of thousands of Syrians seek to make their way to Germany and other wealthy European countries.
Russian airstrikes backing al-Assad's forces have shifted the dynamics of the conflict although the overstretched Syrian troops have so far failed to gain much ground.
Moscow has coupled its intervention with intensified diplomatic efforts, reaching out to the rebels' regional allies.
Russian President Vladimir Putin has made it clear that he has no interest in a permanent military involvement in the region, German Vice Chancellor Sigmar Gabriel said Thursday after meeting Putin in Moscow.
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