"We find it difficult to imagine them at this conference," the official told reporters on condition of anonymity.
UN-Arab League envoy Lakhdar Brahimi met with US and Russian officials today to discuss a guest list for the January 22 peace conference, but he had to acknowledge that the parties failed to resolve the sticky issue of whether Iran could take part.
"Our partners in the United States are still not convinced that Iran's participation would be the right thing," Brahimi told reporters in Geneva, insisting though that Iran's participation was not fully off the table.
"It is no secret that the United States is very concerned that Iran has not been public about its support for the Geneva communique," the senior US official said.
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He also objected to Tehran's military support to Syrian President Bashar al-Assad and its backing of the Lebanese Shiite militia Hezbollah fighting alongside his forces.
Tehran should "think about withdrawing its fighters and its support and allowing the Syrian opposition and the regime to in fact construct a transitional government," the official said.
"No one has said that Iran can't be consulted by the UN, no one has said that Iran can't take actions that would improve the situation," the official said.
Washington, which is also deep in discussions with Tehran on how to implement a landmark deal on the Iranian nuclear programme, would be "happy to see Iran take action, (although) whether that changes our view is a hypothetical question," the official said.
The United States is meanwhile optimistic that Syria's deeply splintered opposition will manage to pull together a delegation to send to the negotiations that is "credible, legitimate and as representative as it can be".