The father and brother of two teenage sisters shot dead in Pakistan after mobile phone video of them with a man surfaced online have been arrested, police said on Monday, drawing praise from women's rights activists.

The man who shot the video has also been arrested, while a relative suspected of carrying out the killing is still at large, local district police officer Shafiullah Gandapur said.

The sisters, who were 16 and 18, were shot dead on Thursday in the remote tribal region of North Waziristan in Khyber Pakhtunkhwa province.

Hundreds of women are killed each year in Pakistan by family members over perceived damage to "honour" that can involve eloping, fraternizing with men or any other infraction against conservative values that govern women's modesty.

Many such killings go unreported, but the 2016 death of social media star Qandeel Baloch at the hands of her brother ignited fierce debate over their prevalence and prompted the government to tighten the laws.

Police have come under mounting pressure to investigate these crimes.

"Our intentions are sincere. We first heard about the incident through social media and decided to confirm it," he told the Thomson Reuters Foundation by phone from North Waziristan.

"We reached the crime scene and found traces of blood as well as a blood-stained fabric. We arrested the brother and father of the two girls who were murdered and today successfully arrested Umar Ayaz, who made the video."

Gulalai Ismail, a Pakistani women's rights activist exiled in the United States, said the swift action by police in filing a case the day after the murders was a "win for tribal women" in the province area.

"In such crimes time is of essence," she said. "And if this is delayed, like seven such murders that happened earlier this month, the incident is swiftly swept under the carpet, with many passed off as suicide or natural deaths."

Human rights experts say enforcement of justice is often lax in cases involving violence against women, with proceedings at times being drawn out while accused killers were freed on bail and cases faded away.

That is particularly true in remote, socially conservative areas like North Waziristan, where women enjoy little freedom and local customs often hold greater sway than federal laws.

"Before 2018, this kind of murder was not considered a crime in the tribal area, neither was it reported," she said. Pakistan's semi-autonomous tribal areas only came under full federal jurisdiction in 2018.

Ismail said a tribal leader had urged locals to punish the teenagers featured in the video after it emerged online.

"In the tribal code of conduct, this punishment for such acts is always murder," she said.

The whereabouts of a third girl who also appeared in the video are unknown, Ismail said, adding: "She needs protection too." (Reporting by Zofeen T. Ebrahim, Editing by Claire Cozens. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers the lives of people around the world who struggle to live freely or fairly.