Asked about a proposal by former U.N. Secretary-General Kofi Annan, backed by Russia and China, that Iran should join diplomatic efforts to resolve the Syrian crisis, White House press secretary Jay Carney said, "I think one would be hard pressed to plausibly suggest that [Iran's involvement] has been a constructive role."
His comment followed up his statement Tuesday that he didn't "think anybody with a straight face could argue that Iran has had a positive impact on developments in Syria."
Iran is one of Syria's few allies.
Annan has been trying to put together a peace deal between insurgents and the regime of Syrian President Bashar Assad, and met in Tehran with Iranian officials this week to garner their support.
But Carney said the United States has an interest in "an outcome that gives the Syrian people the best chance for a brighter future."
"We believe very strongly that President Assad has no place in that transition, because he long ago lost all credibility he might have had and any claim to be a participant in a transition to a democratic future for Syria," Carney said, "We reject that it's likely that Iran could play a constructive role."
The conflict in Syria entered its 16th month in July. As many as 10,000 people have died as a result of the fighting, which the government blames on opposition groups supported by foreigners.