Human Rights Watch on Wednesday accused a controversial Sudanese counter-insurgency unit of having carried out "two sprees of killings and mass rape" of civilians in conflict-riven Darfur since February 2014.
The Rapid Support Forces, a government-backed militia, carried out the abuses during two campaigns against rebels in the western region, the rights watchdog said.
"The RSF led two counter-insurgency campaigns in the long-embattled region of Darfur in 2014 and 2015 in which its forces repeatedly attacked villages, burned and looted homes, beating, raping and executing villagers," HRW charged.
Bashir -- who is wanted by the International Criminal Court (ICC) for alleged war crimes in the region -- announced a new drive to crush the Darfur rebellion in February 2014, called "Decisive Summer".
A follow-up offensive was announced in December, dubbed by Bashir "Decisive Summer 2," and it was during these campaigns that the abuses were committed, according to HRW.
The New York-based watchdog interviewed victims and defectors from the Rapid Support and armed forces.
"Among the most egregious abuses against civilians were torture, extrajudicial killings and mass rapes," the watchdog said.
Other abuses included mass displacements and the destruction of wells and food stores, HRW said, adding that the violations "amount to war crimes".
HRW's UN and crisis advocacy chief Philippe Bolopion said it was "hard to believe that the Sudanese authorities are unaware of what the RSF are doing".
Khartoum sees "the RSF as one of their best tools to fight the rebels", Bolopion said at a press conference in New York.
- 'Killed for resisting' -
One of the worst incidents the watchdog reported was an attack on the rebel-held town of Golo, in the Jebel Marra area straddling North, Central and South Darfur states.
Rapid Support troops took over the town on January 24, killing and raping civilians and burning and looting properties, HRW said based on witness testimonies from 21 people present during the attack.
The interviewees told HRW they had seen "killing, rape, and widespread beating and looting, including the rape of scores of women in Golo's hospital".
"Many of the women were gang raped, often in front of community members who were forced to watch. Some of those who resisted were killed," HRW's report said.
Jebel Marra has been a stronghold for the Sudan Liberation Army-Abdul Wahid, one of the groups battling Bashir since 2003, when mostly black, African rebels mounted a campaign against Khartoum's Arab-dominated government, complaining of marginalisation.
Access to Jebel Marra has been strictly limited even to humanitarian organisations.
HRW Africa director Daniel Bekele said Rapid Support troops had "killed, raped and tortured civilians in scores of villages in an organised, deliberate, and systematic way" during the campaigns, urging Sudan's government to disarm them.
The force was formed to combat rebels in Darfur as well as in South Kordofan and Blue Nile states.
The Rapid Support troops are officially under the control of Sudan's powerful National Intelligence and Security Forces.
In New York, Bolopion urged Britain, France and the United States "to be much more aggressive in the Security Council and push for action against those kinds of abuses that are happening before the eyes of Blue Helmets".
A joint UN-AU peacekeeping forces (UNAMID) was deployed in Darfur in 2007 tasked with protecting civilians and securing aid deliveries.
More than 300,000 people have been killed in Darfur since 2003, the United Nations says, and some 2.5 million forced to flee their homes.
The ICC indicted Bashir in 2009 for alleged war crimes and crimes against humanity and in 2010 on genocide charges related to the Darfur conflict.