Ali Shamkhani, the secretary of Iran's Supreme National Security Council, says Tehran has in "some cases" responded to letters by U.S. President Barack Obama.
Shamkhani made the comments in reaction to a recent report by "The Wall Street Journal" that Obama sent a private letter to Iran's Supreme Leader Ayatollah Ali Khamenei last month about fighting Islamic State (IS) militants.
"The letters by the American president go several years back," Shamkhani was quoted as saying by Iranian media.
He said the U.S. positions were in clear contrast with the contents of the "secret" letters.
Shamkhani didn't provide any more details about the correspondence between the two countries.
U.S. officials have said Washington is not militarily coordinating the fight against IS with Iran.