Syrian opposition leader Moaz Alkhatib said he would accept negotiations with Bashar al-Assad's deputy.
However, he said talks must be based on the principle of the regime's departure.
Mr Alkhatib told al-Arabiya news that he had asked Iran to deliver his negotiations offer to Assad's government.
He was speaking after meeting Iranian, Russian and US officials in Germany.
Meanwhile, two Russians and an Italian have been freed in Syria in exchange for three captured rebels after being held hostage for nearly two months.
The Russian Foreign Ministry said Viktor Gorelov, Abdessattar Hassun and Mario Belluomo had been released on Sunday and were in good health.
They were taken hostage on 12 December last.
"The Russians have already been brought to the Russian embassy in Damascus," the ministry said in a statement on its website.
"An Italian citizen who was kidnapped with them, M Belluomo, will be handed over to the Italian authorities through the Syrian Foreign Ministry."
It said the three had been seized by rebels on the way to the Syrian port of Tartus from the city of Homs.
Italian media have said that Mr Belluomo, who was 63 when kidnapped, had been working in Syria as an engineer at a steel plant near the port city of Latakia.
The Russian Foreign Ministry did not say what the two Russians had been doing in Syria, but Russian media has suggested they also worked at the steel plant.
Russia is Syria's main arms supplier and has long been an ally of President Bashar al-Assad.
It has blocked three UN Security Council resolutions aimed at putting pressure on Mr Assad.
It says his departure must not be a precondition for a negotiated settlement of the 22-month-old conflict that has killed more than 60,000 people.