Pakistan has introduced chemical castration as a possible punishment for serial rapists under a new law that also calls for faster trials of suspected sexual offenders, a government official said on Thursday.

Parliament passed the law on Wednesday and it came into effect immediately, government official Waqar Hussain said, according to a Reuters report.

Prime Minister Imran Khan had said last year he wanted to introduce the penalty amid a serious concern over increasing offences and the specific case of a woman who was dragged out of her car and raped by two men at gunpoint triggering a national outrage.

Fewer than 3% of rapists are convicted in courts in Pakistan, according to the non-profit organisation, War Against Rape.

"Chemical castration is a process duly notified by rules framed by the prime minister, whereby a person is rendered incapable of performing sexual intercourse for any period of his life, as may be determined by the court through administration of drugs which shall be conducted through a notified medical board," according to the bill.

However, Jamaat-i-Islami Senator Mushtaq Ahmed protested over the bill and termed it un-Islamic and against Sharia.

He said a rapist should be hanged publicly, but there was no mention of castration in Sharia.