Iran has been officially invited to participate in the Geneva 2 Syria peace conference on January 22, UN Secretary-General Ban Ki-moon said. The Syrian National Coalition has threatened to withdraw from the talks, unless Iran’s invitation is revoked.

The UN chief also said that he had assurances from Iran's Foreign Minister Mohammad Javad Zarif that Tehran would play a “constructive role" in solving the crisis in Syria, Reuters reports.

“I believe strongly that Iran needs to be part of the solution to the Syrian crisis,” Ban told reporters today in New York after discussions with Iranian officials. “Iran said that they are committed to play a very constructive and important and positive role.”

The US State Department has accepted the invitation of Iran on the condition it expresses “explicit and public support for the full implementation of the Geneva communiqué including the establishment of a transitional governing body by mutual consent with full executive authorities.”

"If Iran does not fully and publicly accept the Geneva communiqué, the invitation must be rescinded," State Department spokeswoman Jen Psaki said, expressing deep concern about Iran's “contributions to the Assad regime's brutal campaign against its own people.”

So far, Tehran has not accepted the Geneva 1 declaration in full because it believes creating a transitional government in Syria would exclude any participation of President Bashar Assad from the country's political future. Ban however said he is convinced Iran eventually will accept the Geneva 1 communique.

"Foreign Minister Zarif and I agree that the goal of the negotiations is to establish, by mutual consent, a transitional governing body with full executive powers," Ban said. "It was on that basis that Foreign Minister Zarif pledged that Iran would play a positive and constructive role in Montreux."

Representatives of Iran and 39 other countries will attend talks in Montreux, Switzerland in advance of negotiations in Geneva starting January 22. Ban also has invited nine other nations on Saturday that have an interest in the Syrian conflict, saying that their presence would be an important display of solidarity. These include representatives of Australia, Bahrain, Belgium, Greece, the Holy See, Luxembourg, Mexico, the Netherlands and the Republic of Korea.

“The Syrian Coalition announces that they will withdraw their attendance in Geneva 2 unless Ban Ki-moon retracts Iran's invitation," Reuters reported, citing Syrian National Coalition spokesman Louay Safi’s statement on Twitter.

According to another senior SNC member, the coalition was “surprised" by the UN chief’s “illogical” move to invite Iran and “cannot in any way accept it.”

Syria's main Western-backed political opposition group earlier agreed to attend the meeting. A powerful group of Syrian Islamist rebels however rejected the upcoming peace talks with the Assad government.

Iran’s Deputy Foreign Minister Hossein Amir Abdollahian said on Saturday that participants of the conference must adopt a “realistic view.”

“Participants in the Geneva 2 forum must adopt a realistic view and remember that their decisions should not lead to the strengthening of extremist movements in Syria,” Abdollahian said during the talks with French Deputy Foreign Minister Jean-Francois Girault, the ISNA news agency reported on Sunday.

“The forum can provide a political solution providing the people of Syria can decide the future of their country… in a democratic solution that will be manifested in their votes,” Abdollahian added.

Geneva 2, initiated by the United States and Russia aims to start a political dialogue and agree transition map to end the nearly three-year war that has killed an estimated 130,000 people and forced millions to flee their homes.

Russia has been lobbying to bring Iran to the negotiations table in Switzerland as the country plays an important role in geopolitical affairs in the region. Tehran is also Damascus’s closest ally.

Washington was the main party opposed to Iran’s participation in Geneva 2 talks, accusing Tehran of providing financial and military aid to the Syrian government and criticizing Tehran for not signing the Geneva 1 communiqué. Washington’s regional allies in the Middle East, Saudi Arabia and Qatar, were also hesitant for Iran to join the diplomatic round in Switzerland.

Iran has repeatedly stated that it is willing to attend the talks without any preconditions. “We insist that Iran will not accept any preconditions to attend Geneva 2,” Foreign Minister Mohammad Javad Zarif reiterated on Saturday.