Burundi's environment minister, Emmanuel Niyonkuru has been shot dead by unknown assailants in the capital Bujumbura, police said.

Niyonkuru, 54, the country's water, environment and planning minister, was "killed by a criminal with a gun on his way home to Rohero, around 00:45," according to a tweet sent by police spokesman Pierre Nkurikiye. He added that a woman has been arrested following the "assassination".

#Burundi Criminalité: Assassinat du Ministre de l'Eau et Environnement : Une femme qui était avec lui arrêtée pour enquête — NKURIKIYE Pierre (@PierreNkurikiye) January 1, 2017

The murder, the first of a serving government minister since Burundi sank into turmoil over President Pierre Nkurunziza's controversial bid for a third term in 2015, comes after months of relative calm.

Also on Twitter, Nkurunziza offered his condolences "to the family and all Burundians", vowing the crime would be punished.

At least 500 people have been killed and 300,000 have fled the country since unrest began in April 2015.