KARACHI: Police on Thursday arrested a man in Karachi for slitting his sister's throat and letting her bleed to death, the latest case of 'honour killing' in the metropolis.

Hayat Khan, aged around 20, used a kitchen knife to murder his 16-year-old sister Sumaira after he found her talking to a man on her mobile phone at their home in the Orangi Town neighbourhood of the city on Wednesday, senior police officer Azfar Mahesar said.

He then threw her out of the house where she continued to bleed and writhe in pain on the steps as neighbours gathered.

Investigating Officer Gulzar Ahmed confirmed the details of the attack adding that Sumaira had suffered multiple stab wounds to the neck, chest and back.

Mobile phone footage shot by an unknown onlooker shows several men and boys standing around Sumaira, who is lying face down, dressed in a red tunic and orange headscarf and draped in a blanket.

Her head is seen lolling from side to side and the fingers of her right hand appear twisted.

A young boy is shown fighting back tears as one man says in Pashto: "Let's take her to a hospital in a car," and another man replies: "It's almost here, it's coming brother."

The video then pans to Hayat, dressed in a purple shirt and blue jeans, who stares directly at the camera.

Abdul Hakeem, a local resident, told AFP: "She was taken by car to a private hospital but she died before she arrived."

Mahesar said police had taken the unusual step of making themselves the complainants in the case, in order to avoid a loophole in the law that allows the relatives of the victim to forgive the killer.

"We have registered the case on behalf of the state as we don't want to give the family the option of settling the case outside court," he said.

The man was taken into custody soon after the incident.

“The arrested suspect told the police that he saw his sister talking to a youngster on the doorstep of their home,” said Mominabad police station SHO Inspector Sabir Khattak.

“He turned suspicious when the youngster ran away after seeing Hayat. He then asked his sister about the youngster but she refused to say anything. In a fit of anger, he took a knife from the kitchen and attacked Sumaira.”

The move came after Inayat Khan, Hayat and Sumaira's father, had told local media "what is done is done" and said he forgave his son.

Hundreds of women are murdered by their relatives in Pakistan each year on the pretext of defending family 'honour'.

"A Girl in the River: The Price of Forgiveness” ─ a film telling the story of a rare survivor of an attempted 'honour killing' ─ won the Academy Award for best documentary short in February.

Pakistan amended its criminal code in 2005 to prevent men who kill female relatives escaping punishment by pardoning themselves as an “heir” of the victim.

But it was left to a judge's discretion to decide whether to impose a prison sentence when other relatives of the victim forgive the killer — a loophole which critics say remains exploited.

Prime Minister Nawaz Sharif vowed to eradicate the "evil" of honour killings in February but no fresh legislation has been tabled since then.