Diplomats preparing for Syrian peace talks next month have failed to agree on whether Iran should attend.
UN-Arab League envoy Lakhdar Brahimi said the United States remained unconvinced about Iran's participation.
Twenty six nations have been asked to join the conference, which is due to be held in Switzerland on 22 January.
The conference aims to end the fighting, arrange the delivery of aid and outline a political transition for Syria.
The BBC reports Mr Brahimi met American and Russian delegations in Geneva to finalise the list of nations taking part.
"On Iran, we haven't agreed yet," he said on Friday. "It's no secret that we in the United Nations welcome the participation of Iran, but our partners in the United States are still not convinced."
America opposes Iran's presence, because of Teheran's support for President Bashar al-Assad.
The fighting in Syria began in March 2011.