United Nations Secretary General Ban Ki-moon said Monday invitations to the next round of Syria peace talks would be sent by the end of year as he launched a fresh appeal for Iran to participate.
"The invitation list is near complete, I hope the question of Iran's participation is resolved soon," Ban told reporters.
"As I have said before, Iran needs to contribute to peace in Syria along with others in the region."
Ban said formal invitations to participate in the talks would be sent "soon, before the end of this month."
Iran, a key backer of the Syrian regime, was not included in a list of participants released by international envoy Lakhdar Brahimi in Geneva on December 20.
The United States remains opposed to the participation of Iran at the conference, which has been scheduled to start on January 22 in the Swiss town of Montreux.
However Ban remains a staunch advocate of Iranian participation, stating on Monday Tehran "can play a very important role" in the peace talks.
"It is a very important regional power. Logically, practically and realistically they should be a part of this meeting," Ban said.
"But unfortunately there are differences of opinion on this matter... We are still trying to address this issue as soon as possible."
Ban meanwhile called on all sides in the Syrian conflict to release hostages, end sieges and allow greater humanitarian access.
He also condemned the "appalling" use of TNT-packed barrel bombs in the northern city of Aleppo, which have left hundreds dead.
Opponents of Syrian President Bashar al-Assad accuse his forces of dropping explosives-filled barrels on rebel-held areas in an effort to demoralize their supporters and turn them against the insurgents.