At least four human rights activists known on social media for their leftist views have gone missing this week, relatives and NGO workers told AFP on Sunday, as analysts voiced rights concerns.

Two of the men — Waqas Goraya and Asim Saeed — disappeared on January 4, according to a cybersecurity NGO, while Salman Haider vanished on Friday and Ahmed Raza Naseer Saturday, relatives said.

The interior ministry has said it will investigate the disappearance of Haider, who is known for his outspoken views on enforced disappearances in Balochistan, but made no reference to the others. All four were active on social media groups.

Know more: Missing persons

Pakistan is routinely ranked among the world's most dangerous for journalists, and reporting critical of the military is considered a major red flag, with journalists at times detained, beaten and even killed.

“The state has controlled TV and now they're focusing on digital spaces,” said Raza Rumi, a writer and analyst who left Pakistan in 2014 after he was attacked by gunmen who shot his driver dead.

A security source denied intelligence services were involved in the disappearances.

Naseer, who suffers from polio, was taken from his family's shop in central Punjab province, his brother Tahir told AFP Sunday.

Hours after Haider was due home Friday evening, his wife received a text message from his phone saying he was leaving his car on the Islamabad expressway, his brother Faizan told AFP.

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Police later found the car and registered a missing persons report. Faizan said his brother had not received any specific threats.

Waqas Goraya, who is usually a resident of the Netherlands, was picked up on January 4, as was Aasim Saeed, said Shahzad Ahmed, head of cyber security NGO Bytes for All.

“None of these activists have been brought to any court of law or levelled with any charges. Their status disappearance is very worrying not only for the families, but also for citizens and larger social media users in the country,” Ahmad said.

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In 2014, when sectarian killings were rife, Salman Haider had penned a poem titled ‘Kafir’, which quickly went viral on social media. The poem critiqued the intolerance prevailing in the country and quickly garnered critical acclaim.

Haider is a lecturer at Fatima Jinnah Women University (FJWU) in Rawalpindi, an actor, writer and a human rights activist, police said, adding that investigators have started examining his social media accounts and e-mail address, as well as combing his mobile phone records.

A case has been registered under Section 365 of the Pakistan Penal Code, which deals with “kidnapping or abducting with intent secretly and wrongfully to confine a person”, at the Loi Bher police station.

Police were also collecting information regarding his activism, the officer said, adding that “although a kidnapping case has been registered, investigators are still not in any position to pinpoint the motive; whether it is a kidnapping or a forced disappearance”.

“An investigation is ongoing and we are thoroughly examining all aspects of the case.”