Iranian Foreign Minister Mohammad Javad Zarif appeared to pull back from his offer of a prisoner swap with the U.S. during an interview with CBS News. Earlier this week Zarif publicly floated the possibility of releasing some of the five Americans imprisoned back to the U.S., but when pressed by "Face the Nation" moderator Margaret Brennan on how the mechanics of such an exchange would work, Zarif said the U.S. first "needs to prove that it's serious" before the two countries can begin negotiations. THE interview will air on Sunday.

Zarif said a number of Iranians are in jail in the U.S. for violating sanctions, and that the U.S. government already knows which prisoners Iran would like in return for American prisoners in Iran being released. He added the caveat that as foreign minister he was only permitted to broker a swap of prisoners and could not simply release any of them as a sign of good faith.

"I am responsible for foreign policy. I'm not responsible for interfering in court's decisions. I can intervene when there is an exchange, an offer of exchange. I cannot intervene as foreign minister," Zarif said.

Earlier this week at the Asia Society in New York, Zarif grabbed headlines by saying he was putting an offer on the table right then to execute a trade. He suggested it was the U.S. standing in the way.

In the interview with Brennan, Zarif denied that Americans imprisoned in Iran were being mistreated. Baquer Namazi, is an 82-year-old Iranian-American and former UN official imprisoned in 2016, although he is currently under house arrest on medical leave. His son, Siamak, an American businessman, is also imprisoned on charges of espionage. The State Department has dismissed those charges as trumped up. His family has asked for the elder Namazi's release due to his failing health.

"He still has to report to Evin Prison. He is still restricted from getting medical care outside of Iran. Why not allow him to leave to get that care?" Brennan asked Zarif, only asking about the prisoners which Iran has acknowledged: Baquer and Siamak Namazi, Xiyue Wang, Karan Vafadari and Michael White. The foreign minister demurred, saying he could not interfere in the Iranian court's decision to imprison Namazi.

"I can intervene as a private individual on humanitarian basis, and I do, I do," he said. "But as foreign minister, I do not have a standing in any Iranian court unless I can engage in an exchange with Iranians who are wrongfully, in our view, detained either inside the United States or elsewhere."

There other U.S. residents being held in Iran, as well as the disappeared former FBI Agent Robert Levinson.

Zarif claimed that Iran had made an offer six months ago to the Trump administration to discuss the status of the prisoners but it went unanswered. He did however acknowledge having received a letter from the U.S. special envoy for Hostage Affairs, Robert O'Brien, requesting their release.

He was dismissive of that request, pointing out O'Briens lower diplomatic rank and characterizing his letter as more of a plea than a negotiation offer. He also objected to referring to the Americans as hostages rather than prisoners.

"We're willing to talk to anybody who's willing to respect Iran and deal with this issue. We don't have anything against Ambassador O'Brien, but we will not deal with the hostage negotiator unless they want us to appoint somebody as our hostage negotiator so that they discuss about Iranian hostages in U.S. jail," Zarif said.

The foreign minister equated Americans being kept in Iran on trumped-up charges to Iranians imprisoned in the U.S. for sanctions violations.

"These are cases where we believe our prisoners are in prison on bogus charges. The United States believes that they are in prison in Iran on bogus charges," Zarif said. He also spoke ominously of the increasing tension between the U.S. and Iran. In recent weeks, the Trump administration designated Iran's elite Revolutionary Guard Corps (IRGC) as a foreign terrorist organization, or "FTO."

That is the first time that the U.S. has ever named part of another government as an FTO, and it is designed to squeeze one of Iran's most powerful military institutions which has deep influence over its economy. Additionally, the U.S. has threatened to sanction other countries who purchase Iranian oil starting on May 2. Iran has threatened to retaliate.

"I have enough responsibility on my shoulders to prevent a war, to try to circumvent U.S. attempts to prevent Iran from engaging in what is legally ours, and that is normal economic relations. So, I do this as a part of my job, as foreign minister to exchange people without attribution of guilt. Simply to make it possible for people to go back home," Zarif said.