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The UK on Wednesday demanded the immediate release of a jailed British-Iranian aid worker who was recently transferred to the mental ward of a Tehran hospital.

Nazanin Zaghari-Ratcliffe's case has created turbulence in the UK's relations with Iran since her 2016 arrest and conviction for allegedly trying to topple Iran's government.

Zaghari-Ratcliffe was transferred from the notorious Evin prison to an isolated mental ward in Imam Khomeini hospital on Monday, where her father confirmed she is being held by the Revolutionary Guard, according to a statement from the Free Nazanin campaign.

"We are extremely concerned about Nazanin's welfare and call for her immediate release," a spokesman for UK Prime Minister Theresa May said according to AFP.

"We urge Iran to allow family members to visit her and check on her care."

Zaghari-Ratcliffe's husband said she was moved to the ward on Monday, where she has been denied access to her father and consular staff.

"The last time she met the Revolutionary Guards, which was when she was on hunger strike, they were pressuring her to sign various denouncements of the British government and to confess to various things... Are they only isolating her again to squeeze her?" her husband said.

"It's possible it's good news. It's possible that it is a prelude to her release and getting treatment, and actually, all my fears are unfounded. And then it's possible that something else is going on."

The 40-year-old aid worker and mother ended a 15-day hunger strike at the end of June.

She was arrested more than three years ago while she was waiting for her flight out of Tehran after taking her then 22-month-old daughter Gabriella to visit her family.

A project manager with the Thomson Reuters Foundation - the global media group's philanthropic arm - Zaghari-Ratcliffe was in Tehran on a private visit to her family, but was convicted of attempting to overthrow the Iranian regime.

She denies those charges.

"I was healthy and happy when I came to Iran to see my parents," Zaghari-Ratcliffe has been quoted as saying by her family.

"Three-and-a-bit years later and I am admitted to a mental health clinic. Look at me now. I ended up in an asylum. It should be an embarrassment."

Read more: UK 'withholds £400m debt payment to Iran' linked to Zaghari-Ratcliffe release

The case has added to long-standing tensions between Tehran and London, which is a major arms supplier to Iran's arch-enemy Saudi Arabia.

Iran confirmed on Tuesday that it had arrested a well-known French-Iranian academic without giving any details of her case.

The detention of Fariba Adelkhah, 60, risks increasing tension between Paris and Tehran at a critical moment in efforts to save a landmark 2015 nuclear deal.

Other Iranian dual nationals jailed in Iran include Iranian-American Siamak Namazi and his father Baquer, who are serving ten-year sentences for espionage in a case that has outraged Washington.

Chinese-American Xiyue Wang, a Princeton University researcher, is serving a 10-year sentence for espionage, and US national Michael White, 46, was this year also sentenced to 10 years.