UN investigators have accused Burundi's government of crimes against humanity and urged the International Criminal Court to open a case "as soon as possible".

The UN Commission of Inquiry on Burundi said on Monday it had "reasonable grounds to believe that crimes against humanity have been committed and continue to be committed" in the country, pointing a finger at "the highest level of the state".

The three investigators, appointed by the UN Human Rights Council last September, described a "climate of fear" fuelled by violations including executions, torture and sexual violence.

Fatsah Ouguergouz, commission chair, told reporters in Geneva that the abuses were part of a "general or systematic attack against the civilian population" that could be considered "state policy".

"We are struck by the scale and the brutality of the violations," Ouguergouz said in a statement.

Given Burundi's record of impunity and the "strong likelihood that the perpetrators of these crimes will remain unpunished," the investigators asked "the International Criminal Court to open an investigation ... as soon as possible".

Burundi's government firmly rejected the allegations, accusing the UN investigators of being "mercenaries" in a Western plot to "enslave African states".

"They are not investigators but mercenaries paid to reinforce a narrative already in circulation in certain Western reports and pave the way for the ICC, this instrument in the hands of the West to enslave African states," presidential spokesman Willy Nyamitwe told AFP news agency.

Nyamitwe went further in a tweet, saying investigators were "paid to reach conclusions that were politically motivated and sponsored by the EU".

#Burundi: Ces personnes agissent comme des mercenaires, payés pour arriver à des conclusions politiquement motivées & commanditées par l'#UE https://t.co/UJgpTZZz1P — Amb. Willy Nyamitwe (@willynyamitwe) September 4, 2017

Last year, Burundi formally announced it was withdrawing from the court, with the move set to take effect on October 27. After that date, the ICC can only open a case if asked to do so by the Security Council.

Burundi plunged into a political crisis in April 2015 when President Pierre Nkurunziza decided to run for a third term that his opponents said was unconstitutional.

He won the elections in July that year, which were boycotted by the opposition.

'Hundreds of victims'

Between 500 and 2,000 people have been killed in clashes in the country, according to United Nations and NGO sources. More than 400,000 people have fled and dozens of opposition activists have been forced into exile.

The commission stressed that the total number of violations is nearly impossible to estimate, but Ouguergouz said the cases of torture and extrajudicial killings ran into the hundreds at least.

The investigators were never permitted to enter Burundi, forcing them to conduct their probe from neighbouring countries, where they interviewed more than 500 victims and witnesses.

About 40 women interviewed for the report recounted being victims of sexual violence, Ouguergouz added.

Accounts from victims, their families and witnesses were rigorously checked and corroborated, said the report.

Nkurunziza himself, surrounded by a close-knit circle of "generals", was behind "big decisions, including ones that led to serious human rights violations", the UN report said.

Burundi suffered a civil war from 1993 until 2006 between majority Hutus and minority Tutsis, which claimed an estimated 300,000 lives.