UN chief Ban Ki-moon has invited Iran to take part in preliminary Syrian peace talks this week in Switzerland, an offer Tehran has accepted, BBC reported.
Mr Ban said he had received assurances that Iran would play a positive role in securing a transitional government.
The preliminary talks will open in Montreux on Wednesday and then continue in Geneva two days later.
Syria's government and the main political opposition group earlier agreed to attend the meeting.
The three-year conflict in Syria has claimed the lives of more than 100,000 people.
An estimated two million people have fled the country and some 6.5 million have been internally displaced.
US reservations
On Sunday, UN Secretary General Ban said that Iran's Foreign Minister Mohammad Javad Zarif had pledged that Tehran would play "a positive and constructive" role in Montreux.
"As I have said repeatedly, I believe strongly that Iran needs to be part of the solution to the Syrian crisis," he added.
And Mr Ban stressed: "Let me be clear - Montreux is not a venue for negotiations. The Syrian parties themselves will begin that process in Geneva on 24 January."
Shortly afterwards, Iran said it accepted the invitation. Tehran had earlier insisted it wanted to take part but without preconditions.
There had been a dispute over whether Iran, a crucial ally of Syrian President Bashar al-Assad, should be taking part in the talks.
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.
Washington is also concerned about Iran's deployment of military personnel in Syria, and its support of Lebanon's Hezbollah movement, which had sent fighters to bolster Mr Assad's forces.
'Courageous vote'
On Saturday, Syria's main political opposition group, the Syrian National Coalition (SNC), agreed to take part in the peace talks.
The coalition's leader, Ahmad Jarba, said the SNC was going to the talks "without any bargain regarding the principles of the revolution and we will not be cheated by Assad's regime".
"The negotiating table for us is a track toward achieving the demands of the revolution - at the top of them, removing the butcher from power," he added.
US Secretary of State John Kerry welcomed the decision.
"This is a courageous vote in the interests of all the Syrian people who have suffered so horribly under the brutality of the Assad regime and a civil war without end," he said in a statement.
UK Foreign Secretary William Hague also praised the SNC's "difficult decision", adding: "As I have said many times, any mutually agreed settlement means that Assad can play no role in Syria's future."
Syrian opposition figures had earlier expressed reluctance to go to Switzerland unless President Assad was excluded from any future transitional government.
Damascus says there cannot be any pre-conditions for the talks.