A 15-year-old girl has been beheaded by her cousin in Kunduz, northern Afghanistan, because he was not allowed to marry her.

Reports suggest the teenager, known only as Gisa, was beheaded by her cousin Sadeq with a knife as she went to collect water from a stream in her village.

Police spokesman Sayed Sarwar Hosseini said Sadeq had repeatedly asked Gisa's family for permission to marry her, but he had been refused each time.

Her father said Gisa was too young to marry, prompting Sadeq to attack Gisa in tandem with with a friend named Massoud.

According to Nigerian newspaper Leadership, Sadeq has confessed to the murder.

"Police arrested Sadeq and his friend Massoud, who had helped him to kill the girl," Hosseini told the newspaper.

In 2009, the government in Afghanistan passed the Elimination of Violence against Women law, which criminalises child marriage, forced marriage, selling and buying women under the pretext of marriage, forced self-immolation and other acts of violence.

However, the country has struggled to apply the law. The UN's high commissioner for human rights, Navi Pillay, said: "Judges, prosecutors and police in many parts of Afghanistan have begun to use the new law, which is a positive development, but unfortunately only in a small percentage of violence against women cases.

"Although the law's implementation is clearly growing, there is a very long way to go before Afghan women are fully protected from violence and their equality is properly upheld through this important law."

Between March 2010 and 2011, the Afghanistan Independent Human Rights Commission registered 2,299 acts of violence against women which contravened the 2009 law.

However, the UN found that just 26 percent of these cases were pursued. Courts filed indictments in just seven per cent of the cases and used the EVAW law as the basis of their judgments in four percent of the total incidents.