The UN-Arab League envoy for Syria, Lakhdar Brahimi, arrived in Tehran on Sunday for talks on the Syria crisis with top Iranian officials, media reports said.
The Fars news agency said he would meet President Hassan Rouhani and Foreign Minister Mohammad Javad Zarif and other officials during his three-day visit.
Shiite Iran is Syria's main regional ally and a staunch supporter of President Bashar al-Assad's regime in its struggle against mainly Sunni rebels backed by Western powers and Arab nations.
Tehran is accused of providing military and financial support to Damascus, despite repeatedly maintaining that it has no official military presence in Syria and that its backing takes the form of humanitarian aid.
Brahimi's arrival coincides with Syria's army seizing total control of the strategic town of Yabrud from rebels after a fierce 48-hour-battle.
UN Secretary General Ban Ki-moon on Friday urged Russia and Iran to encourage Syria to resume peace talks.
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The UN-led negotiations, dubbed Geneva II, broke down on February 15 when a second round ended and no new date was set for them to reconvene.
"Iran is one of the important regional countries who can play an important role, including impressing upon the Syrian authorities to come to the Geneva conference in a more constructive way," Ban said on Friday.
Tehran was excluded from the Geneva II process in January, after Ban reversed a last-minute invitation when Syria's opposition said it would boycott if Iran took part.
The United States and other Western nations say Iran must first support a June 2012 declaration by the major powers calling for a transitional government in Syria before it can play a frontline role in peace talks.
Iran refuses to consent to a transitional government, saying that "the best solution is to organise free and fair elections inside Syria".
Assad has not yet officially announced his candidacy in the presidential election due to be held before July, but he is widely expected to seek another seven-year term.
A new electoral law approved by Syria's parliament on Thursday essentially bars many opposition candidates from standing, including those in the Istanbul-based National Coalition, virtually ensuring Assad's re-election.