The two leaders spoke by phone at the end of Rouhani`s visit to the world`s largest diplomatic gathering last year, but both sides said no call or meeting had been scheduled this time.
“Is it written as a rule somewhere that two presidents must always communicate telephonically?” a smiling Rouhani quipped during a news conference in response to a second question on why Obama had not telephoned him.
“At one point it just so happened… It doesn`t mean that every trip (when) we come here, our departure must culminate in a phone call.”
Obama returned to Washington on Thursday after spending several days in New York drumming up international support for air strikes against jihadists in Iraq and Syria.
Iran, a close ally of Damascus, has criticized the Syria strikes.
Rouhani said telephone calls were “somewhat meaningless” in the bigger picture of striving towards closer Iranian-US relations in the future.
“Because of the sensitivity that still exists between the two countries, such action would only be fruitful,” when done according to a precise plan with clear objectives, the Iranian leader said.
But he sounded a positive note, saying it was not written in stone that Iranian-US relations always had to be driven by conflict.
“One day this will change,” he said. Both sides “must step by step lay a solid foundation on which to build a better future.”