A British-Iranian charity worker held in Tehran for six years said on Monday that the UK government could have helped free her earlier, and called for all "unjustly detained" prisoners in Iran to be released.

Speaking publicly for the first time since returning home, Nazanin Zaghari-Ratcliffe said the UK government knew that Tehran wanted a historic £400-million ($530-million, 480-million euro) debt to be paid in order for her to be liberated.

"I think it was week two or week three that I was arrested, like six years ago, that they (Iran officials) told me, 'We want something off the Brits. We will not let you go until such time that we get it'," she told a news conference.

"And they did keep their promise," said Zaghari-Ratcliffe, who flew home last Wednesday with retired engineer Anoosheh Ashoori, 67, after London agreed to settle the sum paid by the Shah-era Iranian government for tanks in the 1970s, before the Islamic revolution.

She described herself as "a pawn in the hands of two governments" who had been caught up in a wider dispute that had "nothing to do" with her, and said all those unfairly detained in Iran in similar circumstances should be freed.

"The meaning of freedom is never going to be complete (until) such time that all of us who are unjustly detained in Iran are reunited with our families," she added.

Zaghari-Ratcliffe, flanked by her husband, Richard, said little of her experience in prison, including in solitary confinement, but said it will "always haunt" her.

She criticized UK diplomatic efforts over the years to get her out, during which time five foreign ministers promised to secure her release.

"I was told many, many times that 'Oh we're going to get you home'," she said.

"What's happened now should have happened six years ago... I shouldn't have been in prison for six years," she said.

Hunger strike

Another British-Iranian, Morad Tahbaz, who also has a US passport, is still being held in Iran, and his daughter Roxanne also spoke at the news conference.

"He should have been on the same flight and it should happen to the other dual nationals," said Zaghari-Ratcliffe.

Tahbaz's sister said earlier on Monday that he had gone on hunger strike and accused the UK government of abandoning him after the two other detainees were released.

"We've only just found out before we started this afternoon that he's been returned to the prison," Roxanne Tahbaz said.

"Contrary to the public statements that have been made, he's not being reunited with his family. And he certainly has not been given a furlough, as was part of the deal that was presented to us.

"From the outset, we were always assured by the (British foreign ministry) that my father would be included in any deal that was made to release all of the hostages.

Environmental campaigner Tahbaz, in his 60s, was only released on furlough from Tehran's Evin prison on Wednesday and was not allowed to leave the country.

After 48 hours he was taken back to prison, reportedly to have an ankle bracelet fitted, but he has not been heard from since.

"We have heard through a relative just now... that he's been taken from the prison and he's been taken to an undisclosed location and that he's gone on hunger strike," his sister Tarane Tahbaz told BBC radio.

Tarane Tahbaz said that her brother has cancer and had lost 40 kilograms (88 pounds) in weight.

The foreign ministry said "Morad has now been moved from Evin prison to a residential location in Tehran".

"We... continue to lobby the Iranian authorities at the highest levels to allow him to return home immediately, as the Iranian government committed to doing," it added.

Spying

A Tehran court in 2020 jailed Tahbaz for 10 years on charges of spying, conspiring with Washington and damaging national security.

He and seven others convicted on similar charges worked with environmental group Persian Wildlife Heritage Foundation to track endangered species and were arrested on suspicion of espionage in early 2018.

Project manager Zaghari-Ratcliffe worked for the Thomson Reuters Foundation, the philanthropic arm of the news and data agency, and was arrested in Tehran on a visit to family in 2016, accused of plotting to overthrow the regime.

Ashoori, a retired engineer from southeast London, was arrested in 2017 and jailed for 10 years on charges of spying for Israel.

Dual nationals from Austria, Canada, France, Germany, Sweden and the United States have also been arrested in similar circumstances.