A senior US official said on Friday that it was "difficult to imagine" Iran attending Syrian peace talks next month as it had not endorsed the June 2012 communique calling for a transitional government and was providing military support to Damascus.

The official said talks would continue with mediator Lakhdar Brahimi and other states on Iran's participation in the peace talks, but there were many ways for Tehran to take part and this was only the beginning of the process.

"Iran in this instance is the only country that has put its own military people into the fight on the ground, that is a unique position," the senior US official said.

"I would hope that Iran would think about withdrawing its fighters and its support and allowing the Syrian opposition and the regime to in fact construct a transitional government with full executive powers by mutual consent, as Lakhdar Brahimi said, a new Syria.

On Iran, we haven't agreed yet. But it is no secret that we in the United Nations welcome the participation of Iran," Brahimi told a news conference in Geneva.

"But our partners in the United States are still not convinced that Iran's participation would be the right thing to do."

Brahimi said he would continue to work with Iranian officials if they were not officially invited. He said they had told UN Secretary General Ban Ki-moon it would not be the end of the world if they were not present, and they would continue to work with him on the sidelines.

Russian Deputy Foreign Minister Gennady Gatilov told Interfax news agency the Iranian issue might need to be resolved by discussion at ministerial level.

The official also said there were ways for Iran to take part without attending the ministerial-level opening of the talks, but called on Iran to stop sending troops and military funding to pro-government militias including Lebanese Hezbollah in Syria.

Brahimi said he wanted both Syrian sides to release vulnerable prisoners, such as women and children, and to improve humanitarian access as a show of good faith before the Jan. 22 conference opens in Montreux.

"Why doesn't the government unilaterally free the kids that are in prison? Teenagers, quite a few of them, women - there are quite a few of them, old people that were arrested, they were not fighting, they demonstrated, or have been arrested by mistake."

Syria's government has already formed a delegation for the conference, but has yet to give Brahimi the names. The opposition will not have its list of names before Dec. 27, as Brahimi had hoped, but will have it not long after, he said.-Reuters