A video showing a teenage girl being flogged by Taliban fighters has emerged from the Swat Valley in Pakistan, offering a shocking glimpse of militant brutality in the once-peaceful district, and a sign of Taliban influence spreading deeper into the country.

The two-minute video, shot using a mobile phone, shows a burka-clad woman face down on the ground. Two men hold her arms and feet while a third, a black-turbaned fighter with a flowing beard, whips her repeatedly.

"Please stop it," she begs, alternately whimpering or screaming in pain with each blow to the backside. "Either kill me or stop it now."

A crowd of men stands by, watching silently. Off camera a voice issues instructions. "Hold her legs tightly," he says as she squirms and yelps.

After 34 lashes the punishment stops and the wailing woman is led into a stone building, trailed by a Kalashnikov-carrying militant.

Reached by phone, Taliban spokesman Muslim Khan claimed responsibility for the flogging. "She came out of her house with another guy who was not her husband, so we must punish her. There are boundaries you cannot cross," he said. He defended the Taliban's right to thrash women shoppers who were inappropriately dressed, saying it was permitted under Islamic law.

The Guardian received the video through Samar Minallah, a Pashtun documentary maker and anthropologist who lived in Swat for two years in the late 1990s. It has been passed between Swat residents by mobile phones.

Ms Minallah said the punishment had been inflicted within the last 10 days, following the signing of a controversial peace deal under which the provincial government ceded control of the valley's judicial system to the militants.

"This video is being widely circulated because the Taliban want people to see it. They want to give the message that this is taking place after the peace deal because this is something they ideologically believe in," she said.

Local sources including journalists and human rights workers, some of whom declined to be identified, confirmed the video was recent, although estimates of its timing varied between one and three weeks ago. The Taliban spokesman said it predated the peace deal.

Sher Muhammad Khan, an official in Swat with the Human Rights Commission of Pakistan, said: "They have committed so many atrocities since the peace deal. They have taken entire control of the district. There is nobody to control them; they decide disputes according to their whims."

Since the 15 February deal, a hybrid of traditional and Islamic law has been operational in Mingora, the largest city in Swat district. The qazi courts, as they are known, are not operated by the Taliban but by a related political movement. They have a murky legal status because the changes have yet to be signed into law.

Floggings and other physical punishments have not been imposed in Mingora, where some residents have praised the system's efficiency. However, in outlying districts, where government writ has been entirely crushed, a crude form of gun justice prevails.

The woman in the video, named as Chaand and believed to be aged 17, was punished in Matta, a district further up the Swat Valley.

Minallah and other sources said the girl was punished on suspicion of having had an illicit relationship with a married man. She did not receive a trial. "The whole case is based on the suspicions of one neighbour," said Minallah.

The woman's brother is among the men pinning her down, she added. "It's symbolic that he does it with his own hands. It gives him honour in local society, that he has done it for the sake of religion."

The Swat Valley is controlled by Maulana Fazlullah, a charismatic preacher who initially gained popularity through radio broadcasts, then seized control through gun battles, suicide attacks and intimidation of the local population.

Since the peace deal, women have been beaten for shopping unaccompanied in Mingora's main market and dozens of girls' schools remain closed, many of them bombed.

Fazlullah has sworn loyalty to Baitullah Mehsud, the overall Taliban leader from South Waziristan who claimed responsibility for last Monday's eight-hour assault on a police centre in Lahore and has vowed to mount attacks in Washington.

On Wednesday a presumed American drone fired rockets at a compound controlled by his network, killing at least 14 people.

The effective surrender of government authority in Swat has caused great alarm across Pakistan and among western allies.Minallah said she feared Talibanisation would spread across Pakistan. "I have distributed this video because I feel people are in denial. They don't want to believe what is happening."