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That "Geneva 1" accord called for the Syrian government and opposition to form a transitional government "by mutual consent," a phrase Washington says rules out any role for Syrian President Bashar al-Assad. Russia, a sponsor of the plan, disputes that view.
On Monday, U.S. officials said Iran could improve its chances of playing a role in the Syria negotiations by working with Damascus to stop the bombardment of civilians and improve humanitarian access.
"There are ... steps that Iran could take to show the international community that they are serious about playing a positive role," one of the officials said in Brussels.
"Those include calling for an end to the bombardment by the Syrian regime of their own people. It includes calling for and encouraging humanitarian access."
Syrian opposition groups and Washington, which accuse Tehran of supporting Assad with manpower and arms during the uprising against him, have long had reservations about the participation of Iran, although the United Nations special envoy on Syria, Lakhdar Brahimi, has backed Tehran's involvement.
U.N. spokesman Farhan Haq said there was no agreement yet on Iran's role but Secretary-General Ban Ki-moon favors its participation and hopes agreement on that issue can be reached.
About two dozen nations plan to send foreign ministers to a daylong gathering Jan. 22 at a Montreux hotel, with Ban presiding. Syria's warring factions have been invited.
The peace talks will start Jan. 24 at the U.N.'s headquarters in Geneva with meetings between Assad's delegation and Syrian opposition groups. The meetings will be moderated by Brahimi.
Assad's government has said the president will not surrender power and may run again in elections due later this year.