UN peacekeepers have been criticised for working closely with Congo's army
Army troops in the Democratic Republic of Congo shot and beat to death about 50 Rwandans in April and burnt their refugee camp, a UN investigator says.
Philip Alston said about 40 women were also abducted and it prompted a revenge massacre by Rwandan Hutu militia.
His report said military operations this year carried out by the army supported by UN peacekeepers in the east had produced catastrophic results.
They have been pursuing Hutu rebels who have been based in DR Congo for years.
The BBC's Thomas Fessy in the capital, Kinshasa, says Mr Alston gave horrifying details of his investigation.
He said the attack on the makeshift refugee camp of Shalio in North Kivu happened on 27 April.
"Some 40 women were abducted from the camp. A small group of 10 who escaped described being gang-raped, and had severe injuries - some had chunks of their breasts hacked off," AFP news agency quotes him as saying.
The government troops involved were the newly integrated rebels from the Tutsi-led movement which threatened to take over the provincial capital Goma a year ago.
At least 96 civilians were massacred in a neighbouring village by Rwandan-Hutu militia in revenge for the Shalio killings.
DR Congo's Information Minister Lambert Mende said the authorities were aware of the massacre.
But he said they feared repercussions if they arrested former Tutsi rebel commander - Colonel Zimulinda - who Mr Alston alleges orchestrated the Shalio massacre.
"Zimulinda's arrest would have had worse consequences than the crimes of which he is accused," Mr Mende said, according to the Reuters news agency.
In his report, the UN expert said the UN Security Council had transformed the peacekeeping force into a party to the conflict.
Earlier this week a joint report by several international aid agencies said the offensive against the Hutu FDLR rebels in the east had had "disastrous" humanitarian consequences.
Instability has been rife in eastern DR Congo since ethnic Hutus accused of taking part in Rwanda's 1994 genocide fled to the area.
Their presence inflamed ethnic tensions with the local Tutsi community, with rival militias battling one another.