"I said I hope you're going to negotiate, because it's going to be a lot better for Iran," Trump said in the interview with Fox Business Network broadcast Friday.
"I think they want to get that letter. The other alternative is we have to do something, because you can't let another nuclear weapon," the US president said, Reuters reported.
The letter appeared to have been addressed to Leader of the Islamic Revolution Ayatollah Seyed Ali Khamenei.
The White House did not immediately respond to a request about that.