Iraq urged to reveal whereabouts of 643 men and boys disappeared in 2016
The rights watchdog made the appeal as the families of the disappeared persons mark the fifth anniversary of their abduction.
"For five years, the families of these men and boys have been living in anguish, not knowing the fate of their loved ones, or whether they are even alive," said Lynn Maalouf, Amnesty's Deputy Director for the Middle East and North Africa.
"The Iraqi authorities must end this anguish and reveal the fate and whereabouts of those forcibly disappeared by the [PMF]. We also urge the authorities to publicly disclose the findings of their official investigation and hold those responsible to account to the full extent of the law," Maalouf urged.
The forced disappearances took place on 3 June 2016, when thousands of men, women and children fleeing the Saqlawiya area of Anbar province encountered a large group of armed men.
Survivors later identified them as members of the PMF by their flags and uniforms.
The PMF, known in Arabic as the Hashd Al-Shaabi, were fighting alongside Iraq's government to retake the city of Fallujah from the Islamic State group.
The fighters reportedly separated around 1,300 men and boys deemed to be of fighting age from their families. Some 643 were later boarded onto buses and a large truck. The rest of the men were taken to a location known as the "yellow house", where survivors say they were tortured and abused.
Days after the incident, the office of then-Iraqi Prime Minister Haider Al-Abadi formed a committee to investigate abuses and forced disappearances committed during the operations to retake Fallujah. The committee's findings have never been made public.
The PMF, also known as the PMU, is a grouping of mostly Shia militias backed by Iran. The paramilitary force has been accused of severe rights abuses by rights groups and considered by the US to be a key threat to stability in the region.
The government of Prime Minister Mustafa Al-Kadhimi has sided with the US against the groups, however, it has struggled to bring them under control.
Last year, Iraqi forces raided an Iran-backed militia's stronghold in Baghdad and arrested more than a dozen members of the group. The arrested men were let go after unidentified gunmen drove to government buildings in Baghdad's Green Zone to demand their release.