Iran said on Tuesday that American authorities are holding about 20 Iranian nationals in jail, its official news agency reported, a day after Tehran said it was ready for more prisoner swaps with the U.S.

A prisoner exchange over the weekend saw Iran free a Chinese-American scholar from Princeton who had been held for three years on widely criticized espionage charges. It was seen as a rare diplomatic breakthrough between Tehran and Washington after months of tensions.

“We told the American side that our figure is more or less 20 but this is not final,” IRNA quoted as saying Mohsen Baharvand, an aid to Iran's foreign minister. The report didn't provide further details on the claim.

This photo provided by U.S. Embassy Switzerland,  Edward McMullen greets Xiyue Wang in Zurich, Switzerland on Saturday, Dec. 7, 2019.  In a trade conducted in Zurich, Iranian officials handed over Chinese-American graduate student Xiyue Wang, detained in Tehran since 2016, for scientist Massoud Soleimani, who had faced a federal trial in Georgia.  (U.S. Embassy Switzerland via AP)

This photo provided by U.S. Embassy Switzerland, Edward McMullen greets Xiyue Wang in Zurich, Switzerland on Saturday, Dec. 7, 2019. In a trade conducted in Zurich, Iranian officials handed over Chinese-American graduate student Xiyue Wang, detained in Tehran since 2016, for scientist Massoud Soleimani, who had faced a federal trial in Georgia. (U.S. Embassy Switzerland via AP)

According to Baharvand, U.S. authorities say that Iran is holding one American national and five dual U.S.-Iranian nationals. He said Iranian officials didn't have “any orders” so far to begin talks on their release, though he suggested this could be the next “phase” in indirect negotiations between Tehran and Washington.

U.S. citizens held in Iran include U.S. Navy veteran Michael White, who is serving a 10-year espionage sentence, as well as environmentalist Morad Tahbaz, an Iranian with U.S. and British citizenship also initially sentenced to 10 years in prison.

Also in Iran are 83-year-old Baquer Namazi and his son, Siamak Namazi, dual Iranian-American nationals facing 10-year sentences after they were convicted of collaborating with a hostile power. Baquer Namazi now is on a prison furlough. However, the Namazis say he remains unable to leave Iran.

In this photo released on twitter account of Iran's Foreign Minister Mohammad Javad Zarif , Zarif, left, speaks with Iranian scientist Massoud Soleimani onboard a plane while leaving Zurich, Switzerland for Tehran, Iran, Saturday, Dec. 7, 2019. Iran and the U.S. conducted a prisoner exchange Saturday that saw a detained Princeton scholar released for an Iranian scientist held by America, marking a rare diplomatic breakthrough between Tehran and Washington after months of tensions. (Javad Zarif twitter account via AP)

In this photo released on twitter account of Iran's Foreign Minister Mohammad Javad Zarif , Zarif, left, speaks with Iranian scientist Massoud Soleimani onboard a plane while leaving Zurich, Switzerland for Tehran, Iran, Saturday, Dec. 7, 2019. Iran and the U.S. conducted a prisoner exchange Saturday that saw a detained Princeton scholar released for an Iranian scientist held by America, marking a rare diplomatic breakthrough between Tehran and Washington after months of tensions. (Javad Zarif twitter account via AP)

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

Baharvand said Iran didn't undertake direct talks with the U.S. for the weekend's prisoner swap in Switzerland. The two sides only communicated through Swiss officials, who represent U.S. interests in Iran. Washington and Tehran have had no diplomatic ties since the 1979 Islamic revolution.