The Obama administration on Tuesday slammed the idea of Iranian participation in Syrian peace negotiations just as United Nations-Arab League envoy Kofi Annan insisted Tehran’s role would be critical in resolving the growing crisis.

“I don’t think anybody with a straight face could argue that Iran has had a positive impact on developments in Syria,” White House press secretary Jay Carney told reporters on Tuesday.

{mosads}Carney’s comments came hours after Annan discussed his new plan to stop the violence in Syria and transition President Bashar Assad out of power with Iranian Foreign Minister Ali Akbar Salehi.

“Iran has a role to play. And my presence here explains that I believe in that,” Annan told reporters in Tehran after his meeting with Salehi, as reported by Reuters. 

The former U.N. secretary general said he had “received [the] encouragement and cooperation [of] the minister and the [Iranian] government” for his new plan to end the violence in Syria between Assad’s regime and forces seeking his removal from power.

A previous peace plan brokered by Annan in March has largely been considered a failure, particularly after U.N. observers were pulled out of the country after being attacked by Syrian forces.

But on Tuesday, Annan said his new “approach,” which was crafted with Assad during a visit to Damascus on Monday, required help from the Iranian government.

The White House has been generally supportive of Annan’s new roadmap toward peace in Syria, calling for international support of the effort. However, the United States and its allies have vehemently opposed any Iranian participation, primarily due to Tehran’s refusal to open its nuclear program to the international community.

“On the Annan plan, we believe that it is essential that the international community come together behind the plan, that the plan be implemented,” said Carney.

“Syria’s future cannot plausibly have Bashar Assad in the government. He’s long since lost his credibility,” he added. 

However, any participation by Iran in that process will kill any chance the plan has for success, the administration says.

That opposition forced Annan to block Iranian diplomats from attending the latest round of international peace talks on Syria last month.

Along with Iran, Annan also decided not to extend an invitation to representatives from Saudi Arabia to the peace talks held June 30 in Geneva.