Greater efforts must be made to protect girls in Afghanistan, where nearly half are married as child brides and almost one in six weds before they turn 15, the United Nations said Thursday.

"Early marriage is a fundamental violation of human rights and impacts all aspects of a girl's life," a group of UN organisations said in a statement to mark the International Day of the Girl Child.

"Child marriage is defined as any marriage carried out below the age of 18 years, before the girl is psychologically ready to shoulder the responsibilities of marriage and physically prepared to bear children," the statement said.

UN special representative for Afghanistan, Jan Kubis, urged Afghanistan to "ensure that all necessary measures are taken to fulfill its international and national legal obligations in protecting" girls from early marriage.

The UN group, including the children's and women's organisations as well as the Afghan mission, said there was a strong link between the age of a mother and maternal death.

Despite progress in the past 10 years, the UN says Afghanistan's maternal mortality rate is still 327 per 100,000 live births -- one of the worst in the world. "Girls aged 10-14 are five times more likely to die in pregnancy or childbirth than women aged 20-24," the statement said. The UN points to a link between access to education and the prevention of early marriage.

Under the rule of Taliban Islamists in Afghanistan from 1996-2001, girls were not allowed to get an education but official figures show that more than three million are now in school. But in the rural areas, cultural barriers and poor facilities mean that girls still have limited access to education.