Annan spoke to reporters after talks on Tuesday with Foreign Minister Ali Akbar Salehi.
The U.N.-Arab League envoy has been pushing Damascus to withdraw its troops from cities and halt all violence in 48 hours to salvage his peace plan.
Annan said in Tehran that he and his host agreed on the need to "find a peaceful solution to the crisis" and he voiced optimism that the situation on the ground would improve by Thursday morning.
Salehi insisted that "change in Syria" should come under the leadership of Syrian President Bashar Assad.
On Wednesday, activists reported fresh violence in different parts of Syria. The Local Coordination Committees said there was shelling of several rebel-held neighborhoods in the central city of Homs.
The Britain-based Syrian Observatory for Human Rights said "tens of army vehicles" are deploying in the southern town of Maaraba amid intense shooting.
Annan's appeal to Iran comes a day after U.S. Sen. John McCain, R-Ariz., told CBS News correspondent Clarissa Ward in Turkey that he has no doubt whether the peace plan was a failure.
(At left, watch Clarissa Ward's "CBS Evening News" report)
"I think it was a failure from the start. Most of us knew because there was no pressure for Bashar Assad to actually stop the killing. We think it's going to require military action on the ground to get him to leave," McCain said.
McCain and Connecticut independent Sen. Joe Lieberman -- both strong advocates of international intervention -- visited a Syrian refugee camp inside Turkey Tuesday, which was supposed to be the first day of a cease-fire under Annan's peace plan.
Instead, Damascus residents woke up to tanks rolling through their neighborhoods, and shells rained down on the battered city of Homs -- a center of the resistance -- dimming any hope that the Assad regime would honor its commitment to stop the violence.
