"My presence here proves that I believe Iran can play a positive role and should therefore be a part of the solution in the Syrian crisis," Annan said during a news conference with Iranian Foreign Minister Ali-Akbar Salehi.
Annan arrived in Tehran Monday after meeting with Syrian President Bashar Assad in Damascus.
Speaking to reporters on Air Force One en route to Iowa Tuesday, White House press secretary Jay Carney said he doesn't "think anybody with a straight face could argue that Iran has had a positive impact on developments in Syria."
Carney said the United States believes "it is essential" that the international community back the Annan peace plan for Syria "and that the transition that the plan calls for does not include President Assad."
"He will go down in history as a brutal tyrant who murdered his own people," Carney said.
Annan's trip pointed to a widening gulf in international approaches to the Syrian crisis, with U.S. Secretary of State Hillary Clinton saying Monday Assad's "days are numbered," as Annan reached out to the embattled president and his regional partner for a compromise, The New York Times reported Tuesday.
The United States and the European Union have objected to allowing Iran a role in resolving Syria's 16-month crisis. Iranian officials have said elections in 2014 would give the Syrians a chance to elect a new leader through a democratic process, even though it is unclear whether the Syrian opposition would be allowed to participate.
During the news conference Tuesday, Salehi said Annan's comments indicated Iran was "clearly not a part of the problem" in Syria.
Also Tuesday, Russian ships headed to the Syrian port of Tartus to replenish supplies, the Interfax news agency said. A military-diplomatic source told Interfax three overhauled Mi-25 helicopter gunships would eventually be delivered to Syria. The disassembled choppers were aboard a Russian ship that turned back to Russia in June after its British insurer withdrew coverage.
Meanwhile, the Syrian Network for Human Rights and Damascus Center for Human Rights Studies documented at least 81 casualties in different Syrian cities in a report issued Monday.