WASHINGTON: The United States insisted Tuesday that it sees no role for Iran in resolving the conflict in Syria, in an implicit rebuff to UN envoy Kofi Annan’s outreach to Tehran.

Annan left Tehran on Tuesday after a visit to drum up support for his UN and Arab League peace plan for Syria, saying Iran could play a “positive role”in ending a crisis that has cost more than 17,000 lives.

In Washington, officials were skeptical, and insisted the situation will not be resolved until Tehran’s Syrian ally President Bashar al-Assad steps down and allows a negotiated political transition.

“I don't think anybody with a straight face could argue that Iran has had a positive impact on developments in Syria,” White House spokesman Jay Carney told reporters aboard US President Barack Obama’s jet Air Force One.

“We believe it is essential that the international community come together behind the plan, that the plan be implemented, and that the transition that the plan calls for does not include President Assad,” he said.

“We remain highly skeptical about Assad’s willingness to keep his commitments, which is another reason why Syria’s future cannot plausibly have Bashar al-Assad in the government,” he said.

“He’s long since lost his credibility,” he insisted. “Aligning with Bashar al-Assad is aligning with a tyrant and putting your nation on the wrong side of the Syrian people.” The State Department also underscored its concern, and pressured Annan to stand by the West’s tough stance.

State Department spokesman Patrick Ventrell said “broadly speaking, we said Iran has to stop its destructive behavior in Syria. And so, that's obviously part of the... we hope, the joint special envoy delivers in his message.”

Annan, a former UN secretary general, has visited Damascus, Tehran and Baghdad this week to salvage his peace initiative following a meeting of world powers in Geneva late last month, to which Iran was not invited.

The transition plan agreed in Geneva does not make an explicit call for Assad to quit, but Western powers and the Syrian opposition have made it clear they see no role for him in a unity government.