A 20-year-old Afghan woman was beheaded by her mother-in-law with the help of a teenage boy in western Afghanistan, police said.

Abdul Arouf Ahmadi, the police spokesman for Herat province, said the motive was unclear and that Mah Gol was beheaded on October 8.

"The woman's body was thrown in a gutter. The police arrested Najibullah, who had beheaded the woman in Gozara district, on October 15," Ahmadi said.

While the teenager's role was being investigated, police said the whereabouts of the mother-in-law were unknown.

According to local media reports, Gol was killed allegedly by her husband's family for her refusal to become a prostitute.

She was married four months ago, the reports said.

"The tragic fate of Mah Gol is one more incident that highlights the violent atmosphere that women and girls face in Afghanistan and the region," said Suzanne Nossel of Amnesty International.

Earlier this month, an Afghan woman was beheaded by a local militia commander in the northern province of Kunduz. While he was arrested after 10 days, the motive remains unknown.