UN Syria envoy Staffan de Mistura arrived in Tehran on Tuesday for talks with a key backer of President Bashar al-Assad on the eve of fresh peace negotiations in Geneva.
De Mistura will meet Iran's deputy foreign minister Hossein Amir Abdollahian, Iranian media reported, ahead of the latest effort to end five years of fighting that have killed more than 270,000 people.
Iran and Russia have been Assad's top supporters in the conflict, with Tehran providing economic and military support that has backed up his rule.
The conflict has also forced millions to flee their homes, creating a refugee crisis in Europe.
The UN Security Council passed a resolution in December that led to the peace talks. A framework called for Syrian elections to be held 18 months after a transitional government is agreed.
The fate of Assad is a major stumbling block, however.
While the Syrian opposition insists Assad can play no role in a transitional government, the Damascus regime as well as Iran says voters should decide his fate.
De Mistura, a veteran diplomat and negotiator in conflict zones including Iraq and Afghanistan, said Monday when in Damascus that this week's Geneva meetings would be "crucially important".
"We will be focusing in particular on the political transition, on governance and constitutional principles," he told reporters after meeting Syrian Foreign Minister Walid Muallem.
A shaky ceasefire has been in place in Syria since February 27.
The truce, which was brokered by the United States and Russia, does not include areas where the Islamic State (IS) jihadist group and Al-Qaeda's affiliate Al-Nusra Front are present.
Iran's support for Assad has centred on Tehran sending military advisers from its elite Revolutionary Guards to Syria, dozens of whom have been killed.
The Iranian officers have directed militias against rebels fighting Assad's forces.