The one-time Ugandan rebel group Lord's Resistance Army killed at least 321 civilians in a four-day rampage in northeastern Congo late in 2009, Human Rights Watch said early Sunday in Kampala.
The killings took place between December 14 to 17 in the remote Makombo area of Haute Uele district, but had not been reported previously, Human Rights Watch said in a report.
The rights group's investigators documented the killings during a visit to the massacre area in February.
In addition to the killings, the Lord's Resistance Army (LRA) abducted 250 others including at least 80 children, it said.
In keeping with the LRA's brutal reputation of kidnapping and forcing children into its ranks, the group documented cases of some of the 80 children being ordered to murder other children.
Citing accounts by children and adults who managed to escape, the Human Rights watch said the children were ordered to surround the victim in a circle and take turns beating the child on the head with a large wooden stick until the child died.
The LRA was formed in the late 1980s in Uganda. The group was driven out of Uganda in 2002 to the Democratic Republic of Congo and south Sudan from where its members staged cross-border attacks.
In December, the United Nations High Commissioner for Human Rights said that the LRA's systematic killings in Congo "may amount to crimes against humanity".
From September 2008 to June 2009, LRA allegedly killed 1,200 people and abducted 1,400, including 600 children and 400 women, in northeastern Congo. The attacks displaced around 230,000 people, the commissioner's report said.
In 2005, the International Criminal Court issued arrest warrants for LRA chief Joseph Kony and four others for war crimes. Two of the suspects have died in the bush. Kony, Dominic Ongwen and Okot Odhiambo were still being sought.
In early January, Ugandan troops killed a top LRA rebel commander, general Bok Abudema, in the jungles of the Central African Republic.