WASHINGTON - US President Barack Obama disclosed in a television interview broadcast on Sunday that he had exchanged letters with new Iranian President Hassan Rouhani and said diplomacy on Syria, backed up by a military threat, is a potential model for negotiating over Tehran's nuclear ambitions.In an interview on ABC's "This Week with George Stephanopoulos," Obama did not reveal details of the letter exchange, but made clear that US concerns about Iran's nuclear ambitions are a "far larger issue for us" than Syria's chemical weapons.He and Rouhani will speak at the UN General Assembly next week on the same day although there is no plan for them to meet.Obama said Iran should avoid thinking that the United States would not launch a military strike in response to Tehran's nuclear program just because it has not attacked Syria."They shouldn't draw a lesson that we haven't struck, to think we won't strike Iran. On the other hand, what they should draw from this lesson is that there is the potential of resolving these issues diplomatically," said Obama.Tehran denies seeking nuclear weapons but the United States and Israel are working under the assumption that Iran is well along toward developing an atomic weapons program.Regarded as a relative moderate, Rouhani has made conciliatory statements toward Washington since coming to office last month. However, Obama said he doubted Rouhani would "suddenly make it easy" to negotiate with the Iranians."My view is that if you have both a credible threat of force, combined with a rigorous diplomatic effort, that, in fact you can strike a deal," he said.In the interview, Obama rejected Russian President Vladimir Putin's claim that Syrian rebels were responsible for an Aug. 21 chemical gas attack, but he welcomed Putin's diplomatic role in the crisis.Obama and Putin have become unlikely partners on Syria after US threats to launch a military strike over the chemical weapons attack prompted a diplomatic initiative that has led to a deal to secure Syria's poison gas stockpiles."I think there's a way for Mr. Putin, despite me and him having a whole lot of differences, to play an important role in that," Obama said. "And so I welcome him being involved. I welcome him saying, 'I will take responsibility for pushing my client, the Assad regime, to deal with these chemical weapons.'"Obama dismissed Putin's charge that it was the Syrian rebels who launched the chemical weapons attack, rather than forces loyal to Syrian President Bashar Assad, as Washington believes."Well, nobody around the world takes seriously the idea that the rebels were the perpetrators of this," Obama said.Obama defended his handling of the Syria crisis, saying the steps he has taken had led to a situation where Assad has acknowledged he has chemical weapons and that his key ally, Russia, is pressuring Syria to give them up."I think that folks here in Washington like to grade on style," Obama said. "And so had we rolled out something that was very smooth and disciplined and linear, they would have graded it well, even if it was a disastrous policy."