"I am blessed to announce that the nightmare is over, and my son is safely on his way home," Joanne White said in a statement.

The release comes one day after an Iranian scientist, Cyrus Asgari, returned home. He was arrested during an academic visit in 2016 and accused of stealing trade secrets but was cleared last year by a US court.

US officials had insisted that Asgari was not part of a swap, an assertion sure to come into question with White's release.

White, who had served 13 years in the US Navy, was arrested in July 2018 in the northeastern city of Mashhad after visiting a woman whom he had reportedly met online.

He was sentenced the following year to at least 10 years in prison on charges that he insulted Iran's Supreme Leader Ayatollah Ali Khamenei and posted anti-regime remarks on social media under a pseudonym.

In March, as Iran was being hit hard by the coronavirus pandemic, White had been transferred into the custody of Switzerland, which handles US interests in Tehran in the absence of diplomatic relations.

He was flown to the capital Tehran but US officials said that White had been told not to leave Iran.

In her statement, Joanne White gave few details on the release but thanked both the US and Swiss governments.

She also thanked Bill Richardson, a former US ambassador and governor who has often taken up high-profile cases of Americans detained overseas.

Richardson in a statement said he had met with senior Iranian officials including Foreign Minister Mohammad Javad Zarif to win White's release.

President Donald Trump's administration, which has been pushing a "maximum pressure" campaign on Iran that includes sweeping sanctions, in the past has resented efforts by Richardson, a former Democratic presidential candidate.