A fifth social activist has gone missing from Islamabad, a colleague said Wednesday, days after four other campaigners disappeared in a way that has alarmed supporters of free speech.

Samar Abbas, president of the Civil Progressive Alliance of Pakistan (CPAP), an anti-extremism activist group, went missing on Saturday, according to Talib Raza, who worked with him at the Karachi-based organisation.

Abbas's brother, also named Talib, told local media on Wednesday that his brother had vanished over the weekend.

"The family waited for a few days to inform people. When the stories about other activists disappearing started emerging, it became clear what was going on,” Raza told Reuters.

It is not known how the four activists went missing last week, but some rights groups and newspapers are questioning whether state or military agencies were in any way involved.

The interior ministry has said it is doing all it can to recover the missing men.

"This government is not in the business of disappearing people and we will not tolerate such disappearances while we are in power," said Minister for Interior Chaudhry Nisar.

The United Nations and international human rights organisations, including Human Rights Watch, on Wednesday also voiced concerns about the disappearance.

The UN Office of the High Commissioner for Human Rights highlighted in a statement the difficulties surrounding free expression in Pakistan.

Human rights lawyer Jibran Nasir has filed an application with the Supreme Court requesting judicial intervention. The apex court has not yet responded to his request.

"By intimidating and picking up social media activists like this, they have taken away our microphones,” Nasir told Reuters.

Activists held rallies in several major city across Pakistan on Tuesday calling for the recovery of the missing activists.