The French Foreign Ministry said on Tuesday that it was "extremely worried" about the health of a French-Irish citizen held in Iran, as UN rights experts accused Tehran of institutionalised "hostage-taking".
The comments came as an Iranian newspaper said authorities had arrested a German for allegedly photographing oil installations in the south-western province of Khuzestan.
About two dozen foreigners and dual-nationals are detained in Iran, as hopes fade of reviving a 2015 deal on Tehran's nuclear programme and as four months of anti-regime demonstrations rock the country.
Paris is "extremely worried" about the poor health of Bernard Phelan, which "requires appropriate medical monitoring", the ministry said.
Mr Phelan, 64, a travel consultant based in Paris, was arrested in October and is being held in Mashad in north-eastern Iran, Irish media reports say.
Ireland said it had "been providing consular assistance, in close co-ordination with France, since the outset".
His sister, Caroline Masse-Phelan, told AFP he had not been put on trial but was arrested on accusations of spreading anti-regime propaganda.
Ms Masse-Phelan said he was already on a hunger strike and had started to refuse water on Monday.
He is one of seven French nationals held arbitrarily in Iran, the Foreign Ministry said.
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Mr Vandecasteele, 41, was last week handed sentences totalling 40 years on a range of charges, including "espionage".
But with the sentences to run concurrently, he will serve 12 and a half years in jail, the judiciary's Mizan Online website reported. He was also sentenced to 74 lashes.
"We believe Mr Vandecasteele has been arbitrarily deprived of his liberty and is a victim of enforced disappearance," a group of eight independent experts said.
They warned that his "right to a fair trial before an independent and impartial tribunal has been violated".
"These are flagrant violations of Iran's obligations under international law," they said.
The experts called on Tehran to "end the institutionalised practice of hostage-taking and to release the many foreign and dual nationals who remain arbitrarily detained".
Tehran does not recognise dual-nationality for its citizens, and has insisted that all of the foreigners held are on the grounds of domestic law.
Supporters of Mr Vandecasteele and rights groups contend he is being held as part of an attempt by Tehran to pressure Belgium to release an Iranian diplomat jailed for terrorism.
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Meanwhile, Iran's Jam-e Jam newspaper said on Tuesday that authorities had arrested a German national "while taking pictures of oil facilities in Omidiyeh", without providing details on his identity or date of arrest.
Germany said it was aware of the reports and that its ambassador was "currently seeking to shed light on this matter".
Mr Namazi, who was arrested in October 2015, launched the hunger strike on the anniversary of the 2016 release of five other Americans as Iran and the US implemented the landmark nuclear deal brokered by then president Barack Obama.
Mr Obama's successor, Donald Trump, left the deal but obtained the release of two more Americans from Iran in prisoner swaps.
The Biden administration sought to restore the nuclear deal and made it clear it could not go forward without the release of imprisoned Americans.
But Mr Biden has acknowledged that the deal is effectively dead after painstaking negotiations, and as attention focuses on Iran's deadly crackdown on protests.
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Iran has been rocked by demonstrations over the September 16 death in custody of Mahsa Amini, 22, an Iranian Kurd who had been arrested for wearing her hijab "inappropriately".
Authorities have made thousands of arrests amid a bloody crackdown that has claimed hundreds of lives, and have executed four people in connection with the unrest.
The protests have further strained ties between Iran and the West, with Tehran accusing some foreign countries of fomenting what it calls "riots".
French-Irish national Mr Phelan was travelling through Mashad during the protests when he was arrested.
The UN experts said they were "appalled by Akbari's execution", pointing out that he was reportedly tortured and "forced to make false confessions".
He had been arrested more than two years earlier.