Tanzania's highest court has annulled a law which allows families to marry off girls as young as 15 years.

The Supreme Court of Appeal upheld a ruling of a lower court that said child marriages infringed on girls' rights and were unlawful and unconstitutional.

Tanzania's attorney general had appealed against the 2017 High Court ruling, which had struck off sections of the law which allowed the marriage of 15-year-old girls.

The court said girls could not marry before 18 - the age for boys.

The ruling is a milestone for the rights of girls.

According to the UN children's agency, 31% of girls in Tanzania are married before the age of 18, while 5% get married before their 15th birthday.