Rwanda-backed M23 rebels killed at least 140 people in eastern Democratic Republic of Congo in July, Human Rights Watch said in a report on Wednesday that highlighted how violence remains elevated despite the US- and Qatar-backed peace talks underway.

The advocacy group provided new details about the killing spree first reported by Reuters in July. It said total killings in Rutshuru territory in July may exceed 300, echoing similar findings by the United Nations last month.

The report included interviews with civilians that HRW said had survived the killings, elaborating on how the attacks had taken place. One woman, who saw M23 kill her husband with a machete, described being marched all day to a river with about 70 women and children.

"They told us to sit on the edge of the riverbank, and then they started shooting at us," the woman was quoted as saying. She said she survived after falling into the river without being shot.

Another man said that he watched M23 rebels kill his wife and four children aged nine months to 10 years from afar, after failing to make it back in time to save them, according to the report. 

The killings occurred weeks after a June 27 US-brokered preliminary deal between Congo and Rwanda and peace negotiations in Qatar between Congo and M23 rebels.

M23 has previously denied any role in the killings. The group did not respond to a request to comment on the report.

HRW said the witness accounts indicated the attacks took place in at least 14 villages and farming areas near Virunga National Park in eastern Congo in July.

Citing 25 witness accounts, medical staff, UN personnel and other sources, HRW said most victims were ethnic Hutu, with some ethnic Nande. 

HRW urged the UN Security Council, European Union and governments to expand sanctions, press for arrests and prosecutions, and called on Rwanda to allow UN and independent forensic experts into areas under M23 control.   

Reuters could not independently verify HRW's report.