Two European dual citizens will stand trial next week in Iran on undisclosed charges as talks restart to try to revive the 2015 nuclear deal.
Campaigners on behalf of German woman Nahid Taghavi, 66, and Briton Mehran Raoof, 64, said their cases should be highlighted before any new agreement over the nuclear deal is struck with Tehran.
The pair were both arrested on October 16 after a round-up of activists in Tehran and have been held in the city's Evin jail. Their lawyers, trying to discover what charges they face, are being denied access to their case files.
A friend of Mr Raoof, a long-standing labour rights campaigner, said that authorities arrested a number of activists after secretly monitoring political meetings of more than a dozen activists at a Tehran coffee shop.
The pair are due to go on trial on April 28 with a third woman, an Iranian who was arrested on the same night. Supporters were told they will be tried separately.
One of those held in the sweep in October, Arash Johari, was sentenced to 16 years in prison on national security charges, raising fears of harsh sentences for others.
Ms Taghavi was brought before a court last week in preparation for the trial, after being returned to solitary confinement a week earlier.
Daren Nair, of Hostage Aid Worldwide, who is working with the families of both detainees, said Iran was notorious for detaining foreign and dual citizens on trumped-up charges and sentencing them to prison after unfair trials with limited access to lawyers.
"As the [nuclear deal] renegotiations are currently taking place, Hostage Aid Worldwide calls on the German government to work with the international community to free Nahid Taghavi immediately and unconditionally," he said.
The UK and Germany are both signatories to the deal and have been involved in talks seeking to bring the US back into the agreement after former president Donald Trump pulled out in 2018. Senior negotiators say the talks are making progress and will resume next week, but there are still major issues to be resolved.
"I don't understand what they [the German government] are doing," said Ms Taghavi's daughter Mariam Claren. "Is there such low pressure that Iran can do what it wants?"
Iran is accused of seizing foreign citizens without basis to use them as leverage during diplomatic disputes with foreign governments.
Nazanin Zaghari-Ratcliffe, a British-Iranian dual citizen, is still in Iran after completing a five-year jail sentence as a dispute festers between the two countries over an unpaid £400 million ($557m) bill owed by the UK after an aborted arms deal in the 1970s.
Ahmadreza Djalali, a Swedish-Iranian doctor, is under threat of execution for allegedly spying for Israel after he was arrested in 2016. Activists say his fate is tied to Assadollah Assadi, an Iranian diplomat jailed for 20 years in February by a Belgian court for plotting to blow up a dissidents' rally in France.
Ms Taghavi has been held in solitary confinement for more than six months of her detention with limited access to medicines for her high blood pressure. Supporters say her health has worsened.
In a phone call to her daughter today, Ms Taghavi said: "My body is completely damaged ... my skin is dry, sore and feels like leather. I have to sleep on a hard floor with no pillow – the pain in my back is unbearable."
Martin Lessenthin, of the International Society for Human Rights, said: “She is a victim of political hostage-taking and the arbitrary judicial system of Iran. Her isolation and the other conditions of her detention are torture. Germany's government must publicly address this.”
Mr Raoof, 64, who was born and raised in Iran, studied mechanical engineering in Huddersfield, West Yorkshire, before the 1979 revolution.
He later lived in London and now spends most of his time in Tehran working as an English teacher and translator.
He was arrested at his flat in Tehran and officials seized his computer and mobile phone. Mr Raoof has been held in solitary confinement since his arrest.
Satar Rahmani, a UK-based colleague, said: “His lawyer hasn’t been given permission to visit him in prison or access to his documentation. We don’t know what is going to happen.”