The decision was made by judges at the International Criminal Court (ICC) last month just two days before Burundi on October 27 became the first nation to quit the tribunal. But it had been kept under seal until Thursday.
In the decision, the judges authorised ICC chief prosecutor Fatou Bensouda "to open an investigation regarding crimes within the jurisdiction of the court allegedly committed in Burundi or by nationals of Burundi outside Burundi since 26 April 2015 until 26 October 2017."
Bensouda can also extend the probe by her team to acts committed both before and after that date "if certain legal requirements are met".
A violent political crisis was triggered in Burundi when President Pierre Nkurunziza ran for a third term in office, winning July 2015 elections which were boycotted by the opposition.
In a swift reaction, Burundi presidential office spokesman Willy Nyamitwe denounced the "corrupt" ICC in a tweet.
He accused the court of cheating and said it had just "shot itself in the foot."
"Without any doubt, Burundi will emerge the winner of this battle," he added.
Although Burundi officially left the ICC on October 27, the court said it "retains jurisdiction over any crime within its jurisdiction up to and including 26 October, 2017, regardless of Burundi's withdrawal".
jpegMpeg4-1280x720Since Burundi had been a member of the ICC during the period "it has a duty to cooperate with the court for the purpose of this investigation" since it was approved before its "withdrawal became effective," they stressed.
Bensouda opened a preliminary probe in April 2016 into reports of killings, torture and rape amid the political upheavals in the landlocked country in the Great Lakes region.
The judges found there was now "a reasonable basis to proceed with an investigation in relation to crimes against humanity".
The court said according to estimates "at least 1,200 people were allegedly killed, thousands illegally detained, thousands reportedly tortured and hundreds disappeared".
"The alleged acts of violence have reportedly resulted in the displacement of 413,490 persons between April 2015 and May 2017," the judges said.
They warned that if "sufficient evidence" is found the prosecutor could "issue either summonses to appear or warrants of arrest".
According to differing tolls provided by the UN or NGOs, the violence in Burundi has claimed between 500 and 2,000 lives, and more than 400,000 Burundians have fled abroad.
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