UN Secretary General Ban Ki-moon said Wednesday the international organisation has not taken a final decision on whether Iran should attend the Geneva II conference on Syria.
"As of this moment, we have not been able to finalise whether Iran should participate or not," Ban told a press conference in Kuwait where he attended a donors conference for Syria.
"I had consultations with Russia and the United States and there are differences on... the exact role and reasons for this participation," Ban said.
The UN chief said Iran is a very important regional power.
The conference is scheduled to take place next week in Switzerland in a bid to find a political solution to a 34-month civil war that has claimed more than 130,000 lives and displaced millions of people.
Ban also said the Syrian opposition has not yet confirmed whether it will attend the meeting.
"So far, we don't have a confirmation from the Syrian opposition to attend the conference," said Ban, adding that there is not much time left.
He advised the main opposition National Coalition to "come with a coherent delegation" to the conference.
British media reported Tuesday that Britain and the United States have told the main Syrian opposition that they will stop their support if it fails to send a delegation to the peace talks.
"The US and UK are telling us you need to go to Geneva," an unnamed senior Coalition official was quoted as saying by the BBC and the Guardian newspaper.
US Secretary of State John Kerry expressed hope this week that the opposition would attend the so-called Geneva II talks set to open on 22 January, saying they were a "test of credibility of everybody" in the conflict.
The Coalition is expected to make a decision on Friday.