A plane has left the US carrying an Iranian professor after his release from a US prison, Foreign Minister Mohammad Javad Zarif said on Tuesday. "Hello friends. Good news. Dr Sirous Asgari is in the air on a flight back to Iran. Congratulations to his wife and family," Mr Zarif wrote in an Instagram post. In March the minister called for Washington to release Mr Agsari, describing him as one of several Iranian scientists taken “hostage” by the United States. Mr Agsari, a materials science professor from Tehran's Sharif University of Technology, was accused in 2016 of stealing trade secrets in violation of US sanctions on Iran while on an academic visit to Ohio. He was acquitted in November and last month US authorities said the 59-year-old, who tested positive for the novel coronavirus in April, was expected to be deported by the United States once he received medical clearance to leave. His release may come as part of a rare prisoner swap between the US and Iran, despite worsening relations between the two countries since US President Donald Trump exited Iran's 2015 nuclear deal in 2018 and reimposed sanctions on the country that have battered its economy. If a swap goes ahead, Michael White, a US navy veteran who has been detained in Iran since 2018, is a likely candidate to be exchanged. He was arrested in July 2018 and sentenced the following year to 10 years in prison after being convicted of insulting Iran's supreme leader Ayatollah Ali Khamenei and posting on social media under a pseudonym. He was released from prison in mid-March on medical furlough but remains in Iran. Both countries, which hold a number of each other’s nationals, have recently called for them to be released amid the Covid-19 pandemic. Iran is currently battling the worst outbreak in the Middle East, with a surge in cases this week, while the US has reported the largest death toll worldwide from the disease. The Islamic republic is holding at least five Americans, while the US has 19 Iranians in detention, according to a list compiled by <em>AFP</em> based on official statements and media reports. Most of those held in the US are dual nationals and charged with evading sanctions by either exporting goods to Iran or using the US financial system. Americans and dual nationals currently known to be held by Iran include Mr White, Siamak Namazi along with his father Baquer, Morad Tahbaz, Gholam Reza Shahini and Karan Vafadari. Despite having no formal diplomatic ties, the two countries do at times agree to prisoner swaps. Last year, Iran freed US citizen Xiyue Wang, who had been held for three years on spying charges, and the United States released Iranian Massoud Soleimani, who faced charges of violating US sanctions on Iran.