ISLAMABAD—Pakistani authorities ordered an investigation into a video showing a man flogging a screaming woman in the country's northwest where the government recently agreed to introduce Islamic law to end a rebellion by Taliban militants.

The government agreed to allow Islamic law in the Swat Valley under a peace deal struck with a hard-line cleric, sparking criticism from activists who are worried extremists will violate basic human rights, especially those of women. The video that emerged April 3 sparked further criticism, although it appeared to have been made before the peace deal was struck in February.

Sherry Rehman, a former information minister and senior ruling party member, called for the men who participated in the public flogging to be punished, saying they had "crossed a red line" and violated the country's constitution.

"Non-state actors have taken law and justice into their hands in a whole area with a force of their guns," she told the Aaj television network. "They want to enforce a Taliban-style system in the area, and we will not let it happen in Pakistan."

The two-minute video shows a woman in a body-covering burqa face down on the ground with two men holding her arms and feet. A third man in a black turban with a long beard whips her backside repeatedly, causing her to scream repeatedly and shout "Stop it, stop it! It is painful!"

A crowd of men watches silently in the background, and a voice can be heard saying "hold her hands tightly."

It was not exactly clear when the incident took place or who carried it out.

A local Taliban spokesman, Muslim Khan, said the militant group publicly flogged a woman in Swat nine months ago over allegations that she had an illicit relationship with her father-in-law with whom she was living, but he was not sure if the video showed that incident.

Mian Iftikhar Hussein, the information minister for the province that contains Swat, said the incident occurred Jan. 3, without providing additional details. He condemned the flogging and called for those who were involved to be punished.

Khan, the Taliban spokesman, defended the punishment, although he said it should not have been done in public and should have been carried out by a boy who had not yet reached puberty.

Militants have already defied a yearlong military operation in Swat, essentially setting up their own parallel administration in the lush region that once attracted legions of tourists.

President Asif Ali Zardari has yet to sign the bill introducing Islamic law in the valley.