A Saudi humanitarian worker has been under “enforced disappearance” for a year and has reportedly been subject to “severe torture” by the authorities, his sister has said.

Abdulrahman al-Sadhan, 35, was detained without reason in Riyadh in March 2018, Areej al-Sadhan said.

Ms Sadhan, who lives in the US, said her family had heard nothing from her brother since.

Mr Sadhan, a graduate of Notre De Namur University in California, was detained without an arrest warrant at the Red Crescent offices where he worked, his sister said.

She said he had not been granted access to a lawyer, all requests to visit him had been denied and the family still did not know why he had been jailed or if he had been charged.

“He was taken to an unknown place,” she told The Independent. “We only found out he was detained when we did not hear from him. We started to panic after a few days. I would imagine he is in solitary confinement if they do not allow him to contact us.”

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She added: “We have only heard from other detainees and they say he has been subjected to severe torture. We have heard from families of other detainees who have seen him in the prison that he might he die from torture.

“That was when I freaked out and starting calling human rights agencies. We heard this claim twice – first in April and then in October after the Khashoggi killing.

“The other detainees can’t speak much because they are monitored but they were sending a warning message to say, ‘Hey this guy needs help – he is in danger.’ Everyone is super scared to report what they have seen in Saudi Arabia because you can be thrown in jail.”

While growing increasingly anxious, an anonymous caller got in touch with the family, Ms Sadhan said. The caller said officials, believed to be from the secret police, had arrested Mr Sadhan at the Riyadh offices of the Red Crescent.

She added that his neighbours informed the family they saw men dressed in police uniform forcefully entering his house and seizing a laptop, phone, other personal belongings and his car a day after he was detained at his workplace.

Ms Sadhan said they had tried “every possible way” to communicate with Saudi officials about her brother but had heard nothing from them – adding that the family had decided not to speak to the press for a year.

Ms Sadhan, who called for the US government to take action on her brother’s case, has now decided to go public about his case.

“We thought we were being diplomatic and trying to reach him through official ways.

“The last year has been really, really horrible. I miss him terribly. I pray for him to stay strong. He is always in my thoughts and prayers. Hopefully, he will be released and come back safe and sound. I think about him day and night – he is on my mind constantly.

“Waiting makes you think the worst – especially after the scary stories about the women activists being tortured and then the horrible killing of Khashoggi. What are they trying to hide? If they had nothing to hide they would allow us to communicate with him.”

“I had no option but to speak about – it I can’t stay quiet any more. I know speaking out is dangerous but staying silent is even more dangerous.”

She described an “enforced disappearance” as making someone vanish “by force” by locking them up and not allowing them to communicate with the world.

The Sadhan family eventually received confirmation from the authorities that Mr Sadhan had been detained a month after his arrest. Ms Sadhan said the last information they received was in November when the semi-independent National Society for Human Rights told them he had been transferred to a different prison.

Ms Sadhan, who described her brother as a “compassionate” individual with a passion for working with people, said she was not sure why he was detained but thought it could be linked to him criticising human rights violations.

The family assume him to be among the dozens of people arrested in a crackdown on dissent that Crown Prince Mohammed bin Salman has used to strengthen his power over the last three years.

She said she had received threats after sending a tweet which simply asked “Where is my brother?” – adding that she would be informing the FBI and police of the threats and abuse she had been subjected to on Twitter.

“There is no crime in asking where he is,” she said. “But I have had threats to be thrown in the sewer. The threats make me really scared. These are not things to be taken lightly. It is not just an online threat, it could escalate into something serious.”

Ms Sadhan said she had not been back to Saudi Arabia for 10 years and was too scared to return there at the moment – saying it was “too dangerous”.

London-based Saudi rights group, ALQST, said Mr Sadhan’s predicament is one of five cases of “enforced disappearance” that it has documented over the last three years. The other cases involve a preacher, a Syrian pilgrim and two journalists.

ALQST director Yahya Assiri said: “We used to get information through governors or officers in prison but with the current cases we can’t get any information.

“We are expecting they have either killed them or hidden them in a faraway place in Saudi Arabia. We don’t think they are in normal prison. After the Jamal Khashoggi murder and torture of the female activists, the pressure needs to increase on the Saudi government to respect human rights and release people who are under enforced disappearance.”

Saudi Arabia has faced extensive scrutiny and criticism over its human rights record in the wake of its detainment of women’s rights activists. It has also been criticised for its role in the war in Yemen and over the killing of Washington Post columnist Khashoggi in the kingdom’s Istanbul consulate in October.

Eleven women’s rights activists were arrested in May 2018 in Saudi Arabia as part of a crackdown and there have been allegations of torture, claims the authorities have denied.

Earlier in the week, several human rights organisations and news outlets reported Saudi Arabian authorities had released three of the female activists who were imprisoned last year for campaigning to lift the driving ban and dismantle guardianship laws. Under the restrictive laws, women cannot travel without permission from their male guardians and male relatives have a final say over a woman marrying.

Conditions of the women’s release remain unknown and initial reports suggest it is temporary, because their trials continue to move through the criminal court.

Dozens of academics, bloggers, clerics and businessmen have also been rounded up – with ALQST estimating that at least 150 people have been detained over the past three years.