Burundi has released three schoolgirls who were detained for allegedly scribbling on President Pierre Nkurunziza's picture in textbooks, Justice Minister Aimée Laurentine Kanyana has said.

The detention of the girls about two weeks ago on a charge of insulting the head of state had led to a social media campaign under the hashtag #FreeOurGirls, with people posting on Twitter images of the president defaced with clown wigs and twirly images.

The minister said the girls - aged 15,16 and 17— had been provisionally released.

"We call on parents to strengthen the education of their children. We remind children that they have to respect authorities, that the age of criminal responsibility is 15," she told state broadcaster RTNB.