Iran's top diplomat made public Wednesday an offer to the U.S. government for a prisoner swap that he said he made six months ago.

Foreign Minister Mohammad Javad Zarif told the Asia Society that Iran has not yet received a response from the Trump administration.

"If they tell you anything else, they're lying," he said.

The U.S. State Department made no mention of a prisoner swap in a statement which said: "The Iranian regime can demonstrate its seriousness regarding consular issues, including Iranians who have been indicted or convicted of criminal violations of U.S. sanctions laws, by releasing innocent U.S. persons immediately."

Zarif didn't specify who Iran might trade, though he mentioned the case of Nazanin Zaghari-Ratcliffe, a British-Iranian woman detained in Iran for nearly three years.

On the other side, he cited U.S. extradition warrants against an Iranian man with a heart condition held in Germany for trying to buy spare parts for civilian airplanes, and against an Iranian woman imprisoned in Australia for three years who was the translator in the purchase of equipment for Iranian broadcasting. He did not name either of them.

"Now we hear about Nazanin Zahari and her child, and I feel sorry for them and I've done my best to help," Zarif said. "But nobody talks about this lady in Australia who gave birth to a child in prison, whose child is growing up outside prison with his mother in prison."

We believe their charges are phony. - Mohammad Jayad Zarif, Iran's foreign affairs minister

As foreign minister, Zarif said, he can only involve himself on humanitarian grounds and where there is a possibility of a prisoner exchange, which he did once with the United States.

"We believe their charges are phony," he said of Iranians held in the U.S. "The United States believes the charges against these people in Iran are phony.

"And I put this offer on the table publicly now: Exchange them. Let's discuss them. Let's have an exchange. I'm ready to do it, and I have authority to do it."

Several Americans being held in Iran

Last month, former U.S. navy cook Michael R. White from Imperial Beach, Calif., was sentenced to 10 years in prison in Iran, becoming the first American known to be imprisoned there since U.S. President Donald Trump took office.

Michael R. White, an American navy veteran, in an undated photograph. (Joanne White/New York Times)

Washington-based lawyer Mark Zaid told The Associated Press that White was convicted of insulting Iran's supreme leader and posting private information online, but information surrounding the case remained vague.

Three other Americans are known to be held in Iran.

Iranian-American Siamak Namazi and his octogenarian father Baquer, a former representative for UNICEF, who served as governor of Iran's oil-rich Khuzestan province under the U.S.-backed shah, are both serving 10-year sentences on espionage charges.

Iranian-American art dealer Karan Vafadari and his Iranian wife, Afarin Neyssari, received 27-year and 16-year prison sentences, respectively.

Chinese-American graduate student Xiyue Wang was sentenced to 10 years in prison for allegedly "infiltrating" the country while doing doctoral research on Iran's Qajar dynasty.

Iranian-American Robin Shahini was released on bail in 2017 after staging a hunger strike while serving an 18-year prison sentence for "collaboration with a hostile government." Shahini has since returned to America and is now suing Iran in U.S. federal court.

Also in an Iranian prison is Nizar Zakka, a U.S. permanent resident from Lebanon who advocated for internet freedom and has done work for the U.S. government. He was sentenced to 10 years on espionage-related charges.

Former FBI agent Robert Levinson, who vanished in Iran in 2007 while on an unauthorized CIA mission, remains missing. Iran says that Levinson is not in the country and that it has no further information about him, though his family holds Tehran responsible for his disappearance.