PARIS - US Secretary of State John Kerry refused be drawn into a "back and forth" with Iran on Monday following Tehran's assertion that it had rebuffed a US request to coordinate a response to the Islamic State jihadist group."I'm not going to get into a back and forth. I don't want to do that. I don't think that is constructive, frankly," Kerry told reporters at the US ambassador's residence in Paris after an international conference on Iraq attended by 26 countries.Kerry was speaking after Iran's supreme leader, Ayatollah Ali Khamenei, said Tehran had vetoed a US invitation to cooperate in fighting Islamic State. "The American ambassador in Iraq asked our ambassador (in Iraq) for a session to discuss coordinating a fight against Daesh (Islamic State)," said Khamenei, in quotes carried on state news agency IRNA.