US President Barack Obama has secretly written a letter to Iran's Supreme Leader Ayatollah Ali Khamenei to seek his support in the fight against the Islamic State militants.

Obama wrote the letter to Khamenei last month, the Wall Street Journal said. "Mr Obama stressed to Mr Khamenei that any cooperation on Islamic State was largely contingent on Iran reaching a comprehensive agreement with global powers on the future of Tehran's nuclear programme by a November 24 diplomatic deadline, the same people say," said the daily, which based its report on anonymous sources.

This is the fourth letter that Obama has written to the top Iranian leader since coming to power in 2009.