U.S. President Donald Trump had rare positive words for Iran on Saturday, thanking the country for a "very fair" negotiation, successfully pulling off a prisoner swap that saw an American released from Iranian detention amid soaring tensions.

The exchange, which took place in Switzerland, involved a Princeton graduate student jailed in Iran for espionage since 2016 and an Iranian national arrested over a year ago in Chicago.

"Thank you to Iran on a very fair negotiation," tweeted Trump, as Xiyue Wang made his way home to his family. The U.S. leader was expected to welcome Wang in person when he arrives in the United States, after a stop in Germany for medical evaluation.

"It was a one-on-one hostage swap," Trump told reporters. "I think it was great to show than we can do something. It might have been a precursor as to what can be done."

A photo tweeted by the American Embassy in Bern showed Wang on a rain swept tarmac in Zurich with an official blue and white U.S. jet in the background, hugging Ambassador Edward McMullen.

The Chinese-born American was in apparent good health and in "very, very good humor," said a senior US administration official.

Tehran announced the release of Iranian scientist Massoud Soleimani from the United States shortly before Washington declared Wang was returning home. 

State news agency IRNA said Soleimani had been "freed moments ago after one year of illegal detention and was handed over to Iranian officials in Switzerland." 

Iranian Foreign Minister Mohammad Javad Zarif said earlier the prisoner swap was imminent. 

"Glad that Professor Massoud Soleimani and Mr. Xiyue Wang will be joining their families shortly," he tweeted. 

"Many thanks to all engaged, particularly the Swiss government," which has looked after U.S. interests in Iran in the absence of diplomatic ties, Zarif said.

The Swiss foreign ministry confirmed that the exchange – which it called a "humanitarian gesture" – took place on its territory. Both the U.S. and Iran credited Switzerland with an intensive diplomatic effort to secure the men's release.

"Our country stands ready for further facilitation," the foreign ministry statement said.

'Hopeful' sign 

The United States and Iran cut diplomatic ties shortly after the 1979 Islamic Revolution, and relations sharply worsened in May 2018 when Trump withdrew the U.S. from an international accord that gave Iran relief from sanctions in return for curbs on its nuclear program.

The arch-enemies came to the brink of military confrontation in June this year when Iran downed a U.S. drone and Trump ordered retaliatory strikes before cancelling them at the last minute.

Secretary of State Mike Pompeo said the United States was "pleased that Tehran has been constructive in this matter."

Briefing reporters on condition of anonymity, a senior U.S. official noted that Trump "remains committed to talks with Iran without preconditions" – about Tehran's nuclear program, its "malign activities" in the Middle East, and the deadly mass protests that have gripped the country.

While Iran has so far rebuffed U.S. offers of talks, the official said: "We're hopeful that the release of Mr. Wang is a sign that the Iranians may be willing to come to the table to discuss all these issues."

The official also voiced hope that Wang's release signals "the Iranians are realizing that the practice of hostage-taking diplomacy really should come to an end."

Spying allegations

Wang, a Chinese-born American, was serving 10 years on espionage charges in Iran. 

A doctoral candidate in history at Princeton University, he had been researching Iran's Qajar dynasty for his dissertation on late 19th- and early 20th-century Eurasian history when he was imprisoned in August 2016. 

"He was not a spy, he was not involved in espionage, and was wrongfully detained from the start," the U.S. official said.

A statement on the Iranian judiciary's Mizan Online website said Wang had been "freed on Islamic clemency." 

Soleimani, a professor and senior stem cell researcher at Tehran's Tarbiat Modares University, was arrested on arrival at an airport in Chicago on October 22, 2018 for allegedly attempting to ship growth hormones, according to  IRNA. 

The U.S. official confirmed the Justice Department has dropped charges against Soleimani, calling the swap a "reciprocal humanitarian gesture" and a "very, very good deal for the United States."

"There's been absolutely no payments of cash or lifting of sanctions or any sort of concessions or ransom," the official said.

In September, Negar Ghodskani, an Iranian woman sentenced in the United States for violating sanctions against Tehran, was released and returned home after giving birth in custody.

An unknown number of Iranians are detained abroad.

(With input from Reuters, AFP)