President Barack Obama secretly wrote to Iran's Supreme Leader Ayatollah Ali Khamenei in the middle of last month and described a shared interest in fighting Islamic State militants in Iraq and Syria, according to a Wall Street Journal report.
The letter appeared aimed both at buttressing the campaign against Islamic State and nudging Iran's religious leader closer to a nuclear deal.
Citing people briefed on the letter, the WSJ report said it was sent in mid-October, seeking co-operation between the United States and Iran on combating the ISIL, and was tied to a deal being reached between Iran and other nations on Iran's suspected nuclear weapons programme.
Obama administration officials declined to discuss the letter, but "Administration officials didn't deny the letter's existence when questioned by foreign diplomats in recent days," according to the report.
White House spokesman Josh Earnest said on Thursday that US policy towards Iran has not changed, but added that he could not comment on private correspondence between Obama and a world leader.