A report issued by the UN mission in Afghanistan found that although Afghan authorities registered more reports of violence against women under the four-year-old law, prosecutions and convictions remained low.
In a statement, the UN High Commissioner for Human Rights Navi Pillay described the law as a "landmark" and said it "was a huge achievement for all Afghans."
"But implementation has been slow and uneven, with police still reluctant to enforce the legal prohibition against violence and harmful practices, and prosecutors and courts slow to enforce the legal protections in the law," she said.
Women have won back many of the rights they lost during Taliban rule from 1996 to 2001, when the Islamic movement was ousted by an American invasion following the Sept. 11 attacks against the United States. Under the Taliban, girls were barred from attending school, women were forced to stay indoors and cover their heads and faces with burqas.
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There are fears that many of those freedoms may shrink as foreign forces depart by the end of next year and much of the international aid and assistance they brought to Afghanistan goes with them.
But she added that "most of the women victims remain largely unprotected due to a lack of investigation into most incidents and continued underreporting of pervasive violence against women and girls resulting from discrimination, existing social norms and cultural practices, and fear of reprisals and threat to life."
The 49-page report found that incidents of violence against women remain largely under-reported because of cultural restraints, social norms and religious beliefs.
The UN collected information from 18 of Afghanistan's 34 provinces during a 12-month period ending in September to find out how well the law was being implemented.