Iran's Foreign Minister Mohammad Javad Zarif signaled his country's full willingness for a "comprehensive prisoner exchange" with the United States. Zarif made the announcement on his Twitter account Dec. 9, only two days after the Islamic Republic and the United States swapped prisoners in a deal facilitated by the Swiss government, which acts as a neutral intermediary in the absence of formal US-Iran diplomatic ties.

For a long time now, Washington and Tehran have been trading accusations of holding the other side's nationals on a merely political agenda with no solid legal grounds. Suggesting the same, Zarif's tweet described freed Iranian prisoner Masoud Soleimani as a "hostage." A high-profile stem cell scientist, Soleimani was arrested on a research visit to the United States last October. He was formally charged with trying to sidestep US sanctions on Iran in his attempt to import to his home country "growth factors" — proteins used for cell culturing and specifically employed in Soleimani's area of expertise.

In return for the Iranian scientist, the Islamic Republic freed Xiyue Wang, an American student of Chinese origin it convicted of "espionage" and sentenced to a 10-year jail term in 2016. The Princeton University doctoral candidate was flown to the Swiss city of Zurich, where he was handed to Brian Hook, the US State Department's special Iran representative known for his hawkish approach toward the Islamic Republic.

The swap was made possible, according to a New Yorker report, following months of behind-the-scene negotiations involving former US Ambassador to the UN Bill Richardson. The White House has, however, reiterated that the release was "negotiated by current US officials." Iran, on the other hand, has confirmed Zarif-Richardson talks but has also corroborated the White House stance, denying any role played by the former US ambassador, as he carried "no official mandate" to play big in the deal. Mohsen Baharvand, a top Zarif adviser who was closely involved in the exchange process, noted that despite willingness from the American side, there was absolutely no direct negotiation, not even during the brief swap in Switzerland.