US Secretary of State John Kerry met his Iranian counterpart Mohammad Javad Zarif in Vienna on Thursday on the eve of key talks on ending the Syrian conflict.
Foes since Iran's Islamic revolution, Tehran and Washington have no diplomatic ties and the fact that Iran is in Vienna at all will be seen by some as a sign of progress.
Kerry and Zarif know each other well after negotiating a nuclear deal in July, but Washington's Arab allies have resisted allowing Bashar al-Assad's ally a role in the Syria dialogue.
On the eve of broader talks, the top US diplomat left his Vienna hotel and walked 100 yards (metres) up the street to his counterpart's accommodation for a closed-door meeting.
Zarif was also scheduled to meet his Russian counterpart Sergei Lavrov, according to the Russian news wire Interfax, before joining a dozen powers for the main talks on Friday.
The talks come in the context of two days of diplomatic efforts involving all the major international players in the same room for the first time.
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Later Thursday, Russia -- which backs Assad -- will hold talks with the United States, Saudi Arabia and Turkey, all of whom back rebels seeking to overthrow the Syrian leader.
It will be the second time in less than a week that the four envoys have met on Syria.
Friday's talks will be a wider gathering of players which will see Zarif join representatives from Britain, France, Germany, Italy, Egypt, Lebanon and the European Union.
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 step down in favour of a unity government.
Around a quarter of a million people have died in the Syrian conflict over the past four-and-a-half years.