A Chinese-American researcher convicted of spying in Iran has been freed in an exchange for the release of an Iranian scientist held by the US. Xiyue Wang was arrested in Iran in 2016 for "collaborating with foreign governments". Massoud Soleimani, a stem cell expert, was arrested at a Chicago airport last year. He was accused violating trade sanctions by trying to export biological material to Iran. Both denied the charges. The US and Iran have thanked the Swiss government for its assistance as an intermediary facilitator. Mr Wang was flown in a Swiss government airplane from Tehran to Zurich, and then to Ramstein Air Base in Germany, where he will undergo medical check-ups before heading home. Iranian Foreign Minister Mohammad Javad Zarif tweeted to announce the exchange. "Glad that Professor Massoud Soleimani and Mr Xiyue Wang will be joining their families shortly," he wrote. In a statement, US President Donald Trump said Mr Wang had been "held under the pretence of espionage". "Freeing Americans held captive is of vital importance to my Administration, and we will continue to work hard to bring home all our citizens wrongfully held captive overseas," the statement said. Mr Zarif tweeted photos of himself with Mr Soleimani after his release.