The three nations attacked LRA bases in DR Congo in mid-December, forcing the group to flee into surrounding areas.
"At least 900 Congolese have been killed since the beginning of operations, compared to around 40 LRA killed," Dietrich added.
The LRA is also accused of conducting large-scale kidnappings and rapes as well as forced recruitment of minors, all of which has led to a major humanitarian crisis in the region.
"The civilian population are, in effect, caught between the conflicting parties."
UN High Commissioner for Human Rights
Navi Pillay, the UN High Commissioner for Human Rights, described the violations committed in eastern DR Congo by the LRA as "grotesque".
"I'm also concerned that the joint military counter-operations, unless properly planned and executed, could lead to further human rights abuses being perpetrated against the civilian population ... caught between the conflicting parties," she said.
The offensive against the LRA has failed to capture any senior commanders, some of whom are wanted for war crimes by the International Criminal Court (ICC).
Joseph Kony, the LRA's leader, is wanted by the ICC on charges of war crimes and crimes against humanity.
The group began its rebellion against the government of Yoweri Museveni, the Ugandan president, in the 1980s after members of the Acholi community, whose interests the LRA says it is defending, were purged from the army.
LRA violence has left 900 people dead and displaced 130,000 others, with more than 8,000 Congolese taking refuge in Southern Sudan, the UN Office for the Co-ordination of Humanitarian Affairs (OCHA) estimates.