A Burundi court Thursday sentenced four reporters covering an incursion of rebels from neighbouring DR Congo to 18 months in jail, sparking immediate criticism from rights watchdog Amnesty International.

The journalists were working for Iwacu, one of Burundi's few independent media outlets when they were arrested on October 22 and charged with endangering state security.

"We are in shock. The court has just sentenced the four Iwacu journalists to two-and-a-half years in prison and fined them one million Burundi francs (485 euros, $535)," a witness who was in court told AFP.

"It's a shame because they were simply doing their job."

The prosecution had called for a 15-year prison sentence largely based on a WhatsApp exchange of messages between one of the reporters and a colleague based abroad in which the former wrote: "We are heading for Bubanza ... to help the rebels." A further demand was for the four to be denied their civic rights for 20 years.