Washington has said it was "concerned" by reports that the father of a Pakistani activist who fled the country has himself been detained.

Alice Wells, assistant secretary of state for South and Central Asia, tweeted her concern late Thursday over "reports of the continued harassment of Gulalai Ismail's family, and her father's detention today".

She said the US called on Pakistan to "uphold citizens' rights to peaceful assembly, expression, and due process".

Wells tweeted after Ismail said her father Mohammad Ismail had been taken away by unknown men earlier Thursday outside a court in Peshawar.

Rabia Mehmood, a Pakistan researcher for Amnesty International, tweeted that he was in the custody of the cybercrime wing of the Federal Investigation Agency (FIA).

On Friday, Ismail tweeted a copy of what she said was the initial investigation report showing that her father had been accused of "hate speech and false information against Government institutions of Pakistan" on social media.

FIA officials had no immediate comment. A senior security source who spoke to AFP over WhatsApp said: "We (have) nothing to do with this."

Gulalai Ismail is an international award-winning activist who has championed the rights of Pakistani girls in a deeply patriarchal country.

She told AFP during an interview in Washington last month that she fears for her parents, saying they have become socially isolated, with security forces interrogating anyone who so much as texts them.

Mehmood tweeted that the couple had "continuously been harassed by the law enforcement agencies" through late night raids, surveillance and false cases.

A witness told Ismail her father "was dragged into the vehicle while being physically tortured & abused", she tweeted. "I knew they wouldn't spare me... but I hadn't imagined them persecuting my elderly parents," she added later.

Copyright Agence France-Presse, 2019