Iran's Supreme Leader Ayatollah Ali Khamenei has responded to overtures from US President Barack Obama amid nuclear talks by sending him a secret letter, the Wall Street Journal has reported.

Citing an Iranian diplomat, the paper said the Iranian cleric had written to Obama in recent weeks in response to a presidential letter sent in October. Obama's letter suggested the possibility of US-Iranian cooperation in fighting Islamic State if a nuclear deal was secured, the paper said, quoting the diplomat. Khamenei's letter was "respectful" but noncommittal, it quoted the diplomat as saying.