Young supporters of Burundi's ruling CNDD-FDD party have raped women with perceived links to political opponents ever since unrest began to flare there in 2015, Human Rights Watch said Wednesday.
The party's "Imbonerakure" youth wing -- whose name means "The Watchmen" -- has long been accused of using barbaric methods to achieve political ends on behalf of President Pierre Nkurunziza's regime.
"Attackers from Burundi's ruling party youth league tied up, brutally beat, and gang-raped women, often with their children nearby," said Skye Wheeler, women's rights emergencies researcher at Human Rights Watch.
"Many of the women have suffered long-term physical and psychological consequences," Wheeler added.
Nkurunziza's controversial but ultimately successful bid for a third term last year triggered a deadly crisis that has left more than 500 people dead and driven around 270,000 to leave the country.
Several hundred women have reported rapes since then, according to the UN, with the true figures believed to be much higher.
Willy Nyamitwe, a presidential spokesman, said in a series of tweets that the Human Rights Watch publication was "full of errors" and designed to "demonise" his party's youth wing.
"The Imbonerakure are not a gang of rapists. The stigmatisation of @HRW is dangerous and puts their credibility into question," he posted.
Human Rights Watch interviewed more than 70 rape victims in the Nduta refugee camp in western Tanzania who said they had fled following attacks by Imbonerakure.
Testimony revealed the rapes appeared to be motivated by the women's husbands' political affiliations with opposition groups, who were also harassed, beaten and even killed.
"Men armed with guns, sticks, or knives have raped women during attacks on their homes, most often at night," the rights group said, resulting in unwanted pregnancies and the spread of sexually transmitted diseases.
Victims either recognised Imbonerakure foot soldiers, who often control entire towns and villages, or were told that their partner's support for opposition parties was the reason they had been targeted as they were assaulted.
Some women described being raped close to the Tanzania border by men believed to be Imbonerakure, who then "ordered the victims to return home, or verbally harassed them for attempting to leave."
The UN Security Council is under pressure to take action in Burundi, where the descent into violence has raised fears of mass atrocities, similar to those that convulsed neighbouring Rwanda in 1994.
France has presented a draft resolution to the UN Security Council that calls for deploying up to 228 UN police to Burundi to monitor human rights and help quell violence.
Nyamitwe further accused the rights group of being under the orders of "those who are fighting to get their police project passed," a clear reference to France.