Syria's opposition National Coalition on Saturday rejected an Iranian offer to broker talks with the regime, saying the bid was "not serious" and calling Tehran "part of the problem."
"The Iranian initiative is not serious and lacks political credibility," the key opposition grouping said in a statement.
The comments came after Iranian President Hassan Rowhani said Thursday that his government was willing to "facilitate dialogue" between the Syrian government and opposition fighters.
"We must join hands to constructively work toward national dialogue, whether in Syria or Bahrain. We must create an atmosphere where peoples of the region can decide their own fates," Rowhani wrote in a column in The Washington Post.
But the Syrian opposition said it was sceptical that Iran, a staunch ally of President Bashar al-Assad, could broker talks, calling Tehran "part of the problem."
"It would be more useful if the Iranian leadership withdrew its military experts and fighters from Syrian territory," the Coalition said.
The Syrian opposition has long alleged that Iran has sent military personnel to help Assad's forces in their fight against the rebels.
Iran is also a key backer of Lebanon's Shiite Hezbollah movement, which has sent fighters to bolster Syria's armed forces in key battles, notably their recapture of the strategic central town of Qusayr in June.
And Tehran has helped bolster the regime by providing funds to the cash-strapped government.
Rowhani, a moderate on Iran's political scene, has made several diplomatic overtures since his election in June, and there has been speculation that he could meet US President Barack Obama on the sidelines of the UN General Assembly next week.