Sierra Leone's parliament voted on Thursday to significantly increase prison sentences for rape, a move that follows the president's declaration earlier this year that sexual violence constituted a national emergency.

The new legislation sets a 15-year minimum sentence for rape, whereas 15 years was previously the maximum sentence for sex crimes.

Few cases of sexual violence are successfully prosecuted in Sierra Leone, however, part of what activists say is a wider problem of impunity for sexual offenders in many West African countries.

"Fighting sexual offences requires strong punitive measures," said Hindolo Moiwo Gevao, chairman of parliament's legislative committee. "It is necessary we set powerful examples in order to curb the problem."

President Julius Maada Bio declared a national emergency in February after police statistics showed reported cases of sexual and gender-based violence nearly doubling last year. One-third of those involved child victims.

The new law also guarantees free medical treatment for rape victims.