London (AFP) - London demanded the immediate release Wednesday of a jailed British-Iranian aid worker whose husband said she has been transferred to the mental ward of a public hospital in Tehran.

Nazanin Zaghari-Ratcliffe's case has roiled Britain's relations with the Islamic republic since her 2016 arrest and conviction on sedition charges over which she has held a series of hunger strikes.

"We are extremely concrned about Nazanin's welfare and call for her immediate release," Prime Minister Theresa May's spokesman said.

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

UK Foreign Office minister Andrew Murrison said Zaghari-Ratcliffe's husband had told him that she was moved to the ward on Monday.

"It would be indeed cruel to deny this lady, in a psychiatric ward of a public hospital, access to her family. That must happen immediately," Murrison told parliament.

Zaghari-Ratcliffe has described her detention conditions to her family over the phone as "completely contrary to international norms," he added.

- Suicide risk -

The Free Nazanin campaign said she was being held by the Islamic Revolutionary Guard Corps (IRGC) -- a vast and powerful branch of the Iranian military -- at Imam Khomeini hospital.

A psychiatrist has recommended that she be "instantly hospitalised due to her sharp deterioration since her previous meeting, and the risk of her taking matters into her own hands", the campaign group said, in an apparent reference to the possibility of Zaghari-Ratcliffe taking her own life.

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

She was arrested in April 2016 while she was waiting for her flight out of Tehran after taking her then 22-month-old daughter Gabriella to visit her family.

"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."

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Zaghari-Ratcliffe is a project manager with the Thomson Reuters Foundation -- the global media group's philanthropic arm -- but was in Tehran on a private visit to her family.

She was sentenced to five years in jail for allegedly trying to topple the Iranian government.

- 'Compassionate' release -

Zaghari-Ratcliffe's case has added to long-standing tensions between Tehran and London -- a traditional ally and big arms supplier to Iran's arch-enemy Saudi Arabia.

Iranian Foreign Minister Javad Zarif has previously raised the possibility of Zaghari-Ratcliffe being swapped for Iranian nationals held in the United States and Australia.

He told the BBC in an interview aired Wednesday that he also supported the idea of Zaghari-Ratcliffe being released on humanitarian grounds.

"That's not a decision for me to take, but that is an ideal situation for which I have tried and I will continue to try," Zarif said when asked about the possibility of her being "released soon on compassionate grounds".

Western analysts view Zarif as a liberal with limited sway over the much more powerful IRGC and the clerics who surround Ayatollah Ali Khamenei.

"I am not happy to see a single individual in prison and I'll do my best to address that to the maximum of my capacity and my capability," Zarif said.

"But as foreign minister, I am responsible for the conduct of foreign affairs of the country. And she is an Iranian citizen."

Tehran's international isolation has been compounded by its decision to ramp up uranium enrichment in reprisal for Washington's decision last year to pull out of a landmark Iranian nuclear pact.

France and Britain are both signatories to the 2015 nuclear agreement.

The oil-rich Gulf nation confirmed Tuesday it had also arrested French-Iranian academic Fariba Adelkhah, 60.