Human Rights Watch has asked Pakistan to urgently investigate the apparent abductions of four activists who campaigned for human rights and religious freedom, saying their near simultaneous disappearances raised concerns of government involvement.
Tuesday's statement came as left-wing and liberal activists prepared to hold protests throughout the country demanding the bloggers' release.
The missing men are Salman Haider, a poet and academic, and bloggers Waqas Goraya, Aasim Saeed, and Ahmad Raza Naseer.
READ MORE: Concern over fate of missing Pakistani activists
The four went missing from various cities between January 4 and January 7.
"The Pakistani government has an immediate obligation to locate the four missing human rights activists and act to ensure their safety," said Brad Adams, Asia director at Human Rights Watch.
"The nature of these apparent abductions puts the ... government on notice that it can either be part of the solution or it will be held responsible for its role in the problem."
The interior ministry said over the weekend it will investigate the disappearance of Haider, but made no reference to the others.
However, Dawn, the country's leading English language daily, for which Haider has written, issued a strongly worded editorial calling for the activists' immediate release, rather than an investigation.
"The sanitised language - 'missing persons', 'the disappeared', etc - cannot hide an ugly truth: the state of Pakistan continues to be suspected of involvement in the disappearance and illegal detentions of a range of private citizens," it said.
"It is simply not enough for government and police officials to claim that the disappearances are being investigated. Mr Haider and the other recently missing activists need to be returned to their families immediately."
A security source has denied intelligence services were involved in the disappearances, AFP news agency reported.
Protests set to take place
Liberal activists were set to hold protests in major cities on Tuesday afternoon, using the hashtag #RecoverAllActivists to generate support on social media.
Protests were expected in all major cities at press clubs in Karachi, Lahore, Peshawar, Faisalabad and Islamabad, with the protest in Islamabad expected to be the largest, as national media and political officials were expected to join.
Pakistan is routinely ranked among the world's most dangerous countries for journalists, and reporting that is critical of security policies controlled by the powerful military is considered a major red flag, with reporters at times arrested, beaten and even killed.
Rights groups say Pakistani activists and journalists find themselves caught between the country's security establishment and armed groups including the Taliban.
Source: Al Jazeera and news agencies