"The probe into the case is still in the preliminary stage, and information on that can be made public only after technical investigations and interrogation," Press TV quoted the official as saying Friday.
Rezaian, 38, along with his wife Yeganeh Salehi, and two other American freelance journalists were detained in Tehran Tuesday.
Both Rezaian and Salehi has dual US-Iranian nationality.
However, Iran does not recognise dual citizenship.
Rezaian has been the Post's correspondent in Tehran since 2012, and Salehi works as a correspondent for the United Arab Emirates-based newspaper, the National.