US Secretary of State John Kerry will meet his Iranian counterpart in Vienna on Thursday afternoon, ahead of multilateral talks over the Syrian conflict later in the day, a senior State Department official said.
Tehran will take part for the first time in international negotiations aimed at resolving the war, as key nations backing Syria's warring sides seek to thrash out differences over the future of President Bashar al-Assad.
The inclusion of Iran – a key Assad backer – marks a crucial shift after it was excluded from earlier talks, largely due to opposition from the United States and Saudi Arabia.
Iranian Foreign Minister Mohammad Javad Zarif was also due to hold talks on Thursday afternoon with Russia's Sergei Lavrov, according to the Russian news wire Interfax.
The meetings mark the start of a two-day diplomatic flurry which will gather all the major players in the Syrian war in the same room for the first time.
On Thursday evening, Russia – a supporter of Assad – will hold talks with the United States, Saudi Arabia and Turkey, which all back rebel groups seeking to overthrow the Syrian dictator.
It will be the second time in less than a week that the four envoys have met on Syria.
This will be followed by a wider meeting of regional players on Friday, when Zarif will join representatives from Britain, Egypt, France, Germany, Italy, Lebanon and the European Union.
It is the second round of talks in less than a week between envoys from Moscow, Washington, Riyadh and Istanbul.
So far there has been no mention of either the Syrian government or the opposition joining.
The sides remain bitterly divided over the question of Assad, and an earlier round of talks on October 23 ended inconclusively.
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 US and its key regional allies Turkey and Saudi Arabia are supporting groups fighting Assad and insist he must go.
Around a quarter of a million people have died in the Syrian conflict over the past four and a half years.
AFP contributed to this report.