U.N.-Arab League peace envoy Lakhdar Brahimi says 26 nations have been invited to attend a Syria peace conference next month, but that the U.S. has blocked Iran's participation.
Brahimi says the U.S. remains unconvinced that Iran's attendance "would be the right thing to do."
Iran has backed Syria's government in the country's nearly three-year-old conflict.
Brahimi spoke after meeting Friday with U.S. and Russian delegations to determine which nations should be invited to the Syrian peace conference scheduled to open on January 22 in the Swiss city of Montreux.
The conference has been delayed for months due to questions about who should represent the Syrian opposition and government, and which regional powers should be at the table.
The goal of the talks is for the government and opposition to agree on a settlement that includes a transitional authority with full executive powers.
After Friday's meeting, Brahimi also urged Syria's government to halt bomb attacks on civilians and for the Syrian opposition to release all prisoners, including children.
The meeting came a day after Russia blocked a U.N. Security Council statement that would have condemned Syria's government for recent missile and "barrel bomb" attacks on civilians.
The fighting in Syria has killed more than 100,000 people and has forced millions more from their homes.