Iran's Foreign Ministry has advised Iranian nationals not to travel to the U.S. to avoid "arbitrary and long incarceration in completely inhumane conditions".
The Islamic Republic has been seizing and holding dozens of western citizens in prison, usually on dubious charges, without due process since 1980.
Four American citizens are still held in Iran and former FBI agent Robert Levinson, who vanished in Iran in 2007 while on an allegedly unauthorized CIA mission, remains missing as well. Iran denies holding him, but his family says Tehran is responsible for his disappearance.
The statement specifically warns "Iranian scientists to seriously avoid traveling to the U.S. "even if they have invitations for participating in academic conferences”.
On December 7, in a rare act of cooperation Iran swapped Xiyue Wang, an academic who had been detained in Iran for three years on charges of espionage, in return for the release of Massoud Soleinmani, a stem cell expert who was arrested in the U.S. last year for attempting to export biological material to Iran in violation of U.S. sanctions.
Foreign Minister Mahammad-Javad Zarif said on Monday Iran is fully ready for more comprehensive prisoner exchange with the U.S. According to Reuters dozens of Iranians are being held in U.S. prisons, many for violating sanctions. Cabinet spokesman Ali Rabiei on the same day confirmed the Iranian government's wish to cooperate in prisoner exchange. Iran said in October it had given a list of its nationals to the U.S. for exchange.
On December 8 in an interview with CBS the National Security Adviser of the White House said the Trump administration is continuing its efforts to bring all American prisoners "wrongfully detained" home through consular dialogue with Iran.
Iran and the U.S. have for some time been negotiating through Switzerland which represents the interest of the U.S. in Iran. The talks led to the release of Nizar Zakka, a Lebanese businessman with U.S. permanent residency after four years in prison in Iran as a "goodwill gesture" in June.