Fighters from Democratic Republic of Congo's M23 rebel group killed at least 20 men and raped scores of women and girls in the east in November, Amnesty International said in a report on Friday.
The militia, which stepped up its offensive in regions near the borders of Rwanda and Uganda last year, dismissed the accusations, saying they were part of a smear campaign.
The reported death toll was much lower than estimates from the United Nations, which said in December the rebels executed at least 131 people in reprisal killings in a campaign of murder, rape and looting in Kishishe and Bambo villages.
Amnesty said it interviewed survivors and witnesses who described "groups of M23 fighters going house-to-house in Kishishe, summarily killing every adult male they found and subjecting scores of women to rape, including gang rape".
Based on that testimony, Amnesty estimated the M23 killed at least 20 men and raped at least 66 women and girls, mainly in Kishishe, between Nov. 21-30.
M23 spokesperson Willy Ngoma dismissed the report. "There were no serious investigations because they did not come to the field," he told Reuters, adding that none of his soldiers would commit rape.
Congo's government spokesman did not immediately respond to a request for comment. The U.N. peacekeeping mission in eastern Congo also did not respond to a request for comment.
Amnesty said its findings were based on research conducted on the ground in M23-occupied territory.
Congo's army has been locked in heavy fighting since May last year with M23, which is waging its most sustained offensive since a 2012-2013 insurrection that seized vast swathes of territory.
The conflict has sparked a diplomatic crisis between Congo and neighbouring Rwanda, which Kinshasa accuses of backing the rebels, including by sending its own troops into eastern Congo. Rwanda denies any involvement.
The M23 and its predecessor groups have claimed to defend Tutsi interests against ethnic Hutu militias whose leaders participated in the 1994 Rwandan genocide of more than 800,000 Tutsis and moderate Hutus.
Amnesty called on the Congolese authorities to follow through on a pledge to investigate the alleged crimes and hold the perpetrators to account.
"The scale and brutality of these mass rapes is particularly shocking. M23's actions in the Kishishe area constitute war crimes," it said.
(Reporting by Alessandra Prentice; Additional reporting by Sonia Rolley; Editing by Nellie Peyton and Andrew Heavens)
