US President Barack Obama has exchanged letters with President Hasan Rouhani of Iran. The disclosure was made in an interview on ABC TV.
"I think this new president is not going to suddenly make it easy," Mr Obama said on Sunday.
The BBC reports Mr Obama was referring to the dispute over Iran's nuclear programme, which Western countries suspect is aimed at acquiring a nuclear weapon.
Iran denies this and says the programme's aims are purely peaceful.
Mr Obama said in the interview that Iran should not take comfort from the fact that the United States had not taken military action in Syria.
"What they should draw from this lesson is that there is the potential of resolving these issues diplomatically," Mr Obama said.
"If you have both a credible threat of force, combined with a rigorous diplomatic effort… you can strike a deal," he went on.
The BBC reports Mr Rouhani is seen as trying to build a more conciliatory foreign policy than his predecessor Mahmoud Ahmadinejad.