Three teenage schoolgirls in Burundi have been sent to prison to await trial for scribbling on a picture of President Pierre Nkurunziza in textbooks, activists said Thursday.

The Burundi government has ordered the United Nations Human Rights Council to shut its office in the country within two months, foreign ministry and UN sources told AFP.

UN investigators on Wednesday warned that crimes against humanity and other serious rights violations were continuing unabated in Burundi, blaming in part President Pierre Nkurunziza's repeated calls to hatred and violence.

President Pierre Nkurunziza of Burundi on Thursday enacted a new constitution adopted by a referendum, despite opposition concern that the charter will enable him to keep his grip on power until 2034.

The coalition led by main opposition leader Agathon Rwasa said Saturday it did not recognise the result of a referendum on constitutional reforms that could leave President Pierre Nkurunziza in power until 2034.

Burundi President Pierre Nkurunziza is an evangelical who believes he was chosen by God to rule the east African nation, and plans to do so for as long as possible.

Burundi's press regulator on Friday said it was suspending broadcasts by the BBC and Voice of America (VOA) by local radio stations ahead of a constitutional referendum on May 17.

Burundi's influential Catholic bishops said Thursday that they were opposed to fundamental constitutional changes to be voted on in a referendum this month.

Burundi on Tuesday launched the official campaign for a referendum on constitutional change that could enable President Pierre Nkurunziza to stay in power for another 16 years.

Police in Burundi have threatened to arrest anyone campaigning prematurely against a referendum to revise the constitution, which if passed could allow President Pierre Nkurunziza to remain in power until 2034.

Burundian President Pierre Nkurunziza has launched a campaign to promote a referendum to change the constitution, a move that could see him rule until 2034.

A new round of talks aimed at ending Burundi's lengthy political crisis began Tuesday in Arusha, Tanzania, though the negotiations were marked by the main opposition group's absence.