A humanitarian organization has accused Rwandan-backed rebels of massacring civilians in the eastern Rutshuru district of the Democratic Republic of Congo.

In a report published on Wednesday, Human Rights Watch—an international NGO that conducts research and advocacy on human rights—said more than 140 people were killed by Congolese rebels in July. The March 23 Movement, known as M23, is a Rwandan-backed paramilitary organization that has waged an armed struggle against the government since 2012.

Based on eyewitness testimonies, the Human Rights Watch report says that the total number of people killed in the Rutshuru region may exceed 3,000, supporting findings by the UN last month.

The witnesses allege that attacks occurred in at least 14 villages and farming areas near the Virunga National Park, with most of the victims being ethnic Hutu and some ethnic Nande peoples.

Human Rights Watch has appealed to the UN Security Council, the European Union, and governments “to expand sanctions and press for arrests and prosecutions” in response to the killings, and called on Rwanda to allow “UN and independent forensic experts” into areas under M23 control.”

Earlier this week, M23 left peace talks with the Congolese government, saying the government has not adhered to a ceasefire deal. The government has denied the charges, and claimed in response that M23 is violating the agreement. By Christopher Wells, Vatican