U.S. Secretary of State John Kerry says the issue of four Americans still detained in Iran was brought up by the U.S. side during every meeting at the recent Iran nuclear talks in Vienna.

"There was not a meeting that took place... where we did not raise the issue of our American citizens being held," Kerry told MSNBC.  

Kerry stressed that the issue was even discussed during his final meeting with his Iranian counterpart Mohammad Javad Zarif in Vienna, before the historic Iran nuclear agreement was announced.

"We remain very, very hopeful that Iran will make a decision to do the right thing and return those citizens to the United States," Kerry said.