Russian Foreign Minister Sergei Lavrov will hold talks with Syrian opposition representatives on Friday, Moscow said, but there were no details on which groups will attend.
The meeting comes after two days of Russian-brokered talks between the Syrian regime and armed rebel groups in Kazakhstan ended on Tuesday without a major breakthrough.
A spokeswoman for the Russian foreign ministry confirmed the meeting but could not say which groups from Syria's dizzying array of opposition would be represented there.
A rebel negotiator from the armed opposition delegation that attended the Astana talks said they had not yet been invited to attend the meeting with Lavrov, but did not rule out heading to Moscow if they were asked.
"We did not receive an invitation," Fares Buyush told AFP from Istanbul after the rebels left Astana.
"The problem isn't the invitation, it's the topic of discussion. If it's serious and we'll be discussing a national issue, we'll go to the end of the world," Buyush said.
Key players Russia, Iran and Turkey agreed in Astana to establish a joint "mechanism" to shore up a shaky three-week truce in Syria, but offered few concrete details on how it would work.
Representatives from Damascus and the armed rebels in the Kazakh capital were expected to hold their first face-to-face talks since the conflict erupted in 2011, but the rebels refused amid a bitter war of words and mediators had to shuttle between the two sides.
Russia has sidelined the West with its new diplomatic push to find a political settlement to the war in Syria, after its military intervention to support leader Bashar al-Assad turned the tables on the battlefield.
Rebel forces are reeling after regime troops, backed by Russian and Iranian firepower, dealt them the biggest blow in over five years of fighting by capturing their stronghold in eastern Aleppo last month.
The war in Syria has cost some 310,000 lives and forced millions to flee the country since Assad's brutal crackdown on protesters turned into an armed conflict in 2011.
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