Story highlights A plane with $400 million cash was held up over concern Iran would not come through on its agreement

State Department John Kirby confirmed the details publicly Thursday

(CNN) The US used a plane filled with $400 million in cash intended for Iran as "leverage" to ensure that American prisoners were released by Tehran, a State Department spokesman said Thursday.

The money was held up because of concern Iran would not come through on its agreement to release the prisoners earlier this year, spokesman John Kirby said.

Kirby insisted the money was not a quid pro quo for the hostage release, explaining that Tehran ultimately would have received the money, separate from the prisoner release.

"First of all, this was Iran's money," Kirby told CNN's Kate Bolduan on "OutFront. "It was money that was awarded to them by the Hague tribunal and it was a process that had been working independently for many months prior to that."

He continued, "But when the lines all came together, right there in January in a very short 24 hour or so period, it would have been foolish, we believe, for us not to hold on just a little bit longer to make sure that we had the right leverage to get those Americans out and that was the priority."

Read More