Opposition lawmakers in Pakistan have called for an investigation into the disappearance last week of four activists, including a prominent university professor who has been critical of the Taliban and of the government's efforts against militants.

Five Pakistan People's Party (PPP) lawmakers raised concerns in parliament on January 9 over the disappearance of poet and rights activist Salman Haider and three others.

"The pattern of these disappearances suggests that it is a planned and coordinated action undertaken to silence voices which are critical of prevalent sociopolitical issues in Pakistan," the lawmakers of the left-leaning PPP said without specifically implicating anyone.

Haider disappeared on January 6 in Islamabad, his brother said. Last year, Haider wrote a poem about human rights abuses in Pakistan's southwestern Balochistan Province and questioned whether he might become a victim himself.

He had also participated in protests against disappearances of nationalists and separatists from Balochistan. The fate of many of these people remains unknown.

An Interior Ministry spokesman said that "everything is being done to recover Salman." He did not respond to requests for comment about the other activists, Reuters reported.

Based on reporting by Reuters and AP