Pakistani police have arrested a woman for allegedly plotting with her newly married niece to poison the younger woman's husband with tainted milk which would eventually kill him and 17 other relatives in a remote village.

Investigators believe the boyfriend of 21-year-old Aasia Bibi incited her to kill her husband, Mohammad Amjad, by putting rat poison in his milk because she was not happy in her arranged marriage. Mr Amjad was her cousin.

Local police chief Zulfiqar Ali said Amjad did not drink the milk Bibi prepared the night of October 24, but it was later found by his mother, 52-year-old Janat Bibi, who unwittingly used the tainted milk to make a traditional yoghurt drink the following day in the village south of Multan in central Pakistan.

The yoghurt was served to 27 members of the extended family, including Amjad, who along with 17 others who subsequently died at a district hospital. Eight children aged between seven and 12 were among the dead.

Mr Ali said the deaths quickly drew the attention of police, who uncovered an alleged plot involving Aasia Bibi, her boyfriend Shahid Lashari and her aunt, 49-year-old Zarina Begum.

The police chief said Bibi and her boyfriend were arrested and detained pending trial after they confessed to their involvement in the poisoning.

Among the dead were Mr Amjad's two brothers, his three sisters-in-law and some distant relatives.

Mr Ali said: "The family of Amjad did not contact police after this tragic incident but we took the initiative and launched a probe into the matter when bodies started coming to the village from the hospital."

He said Bibi was among those who did not drink the traditional Lassi, which is made with water and yoghurt.

"Her husband was in a critical condition at a hospital and she looked as if nothing had happened and she was cool and calm at her home, and it raised suspicions," he added.

Mr Ali said police first arrested Lashari and he quickly confessed to his role in supplying the rat poison to his girlfriend.

He said the man also told officers that Bibi's aunt, Zareena Begum, used to arrange for the couple to meet at her home and she was aware of the plot to kill Mr Amjad.

Mr Ali said that before detaining Bibi, police collected her mobile phone data, enabling investigators to surmise that she was in constant contact with Lashari after allegedly poisoning her husband's milk.

He said Bibi confessed to her role in the killings upon seeing her boyfriend in handcuffs at a police station.

Another senior police officer, Sohail Habib Tajak, said Bibi was unhappy over her arranged marriage and that she had warned her parents that she was capable of going to any length to get rid of her husband.

Zohra Yousaf, a top human rights activist based in Karachi, said Bibi is among countless women who are forced by their parents to marry against their wishes but that it is rare for a wife to kill her husband.

She said Bibi's alleged actions reveal that she suffers from depression and anxiety.

Many parents in Pakistan arrange marriages for their daughters against their will, and nearly 1,000 Pakistani women are killed by close relatives each year in so-called "honour killings" for marrying against the consent of their family, or attempting to flee the unions.