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United Nations secretary-general Ban Ki-moon has invited Iran to take part in preliminary Syrian peace talks in Switzerland.
Ban said he had received assurances that Iran would play a positive role in securing a transitional government, as Tehran accepted the offer to participate in the talks.
He added that Iranian Foreign Minister Mohammad Javad Zarif had pledged that Tehran would play 'a positive and constructive' role in the talks.
According to the BBC, shortly after Iran accepted the offer, Syria's main opposition group said it would withdraw from the talks unless Ban retracted the offer to Tehran.
The United States said the offer must be conditional on Iran's support for the 2012 deal on Syria's transition.
The preliminary talks will open in Montreux on Wednesday and then continue in Geneva two days later.
There had been a dispute over whether Iran, a crucial ally of Syrian President Bashar al-Assad, should be taking part in the talks.
According to the report, the UN and Russia had advocated a role for Tehran, but the US had reservations because of its failure to endorse the 2012 Geneva communique, detailing Syria's political transition process.
