Girls were believed to have been raped and decapitated while a child was also dismembered in a wave of attacks in North Kivu last week, MONUSCO said in a statement.
A local civil society organisation blamed the rampage on the Muslim Ugandan rebel group ADF-Nalu, one of the oldest but least known armed organisations based in the Democratic Republic of Congo.
"The victims, including women and children -- the youngest of which is thought to have been only a few months old -- were hacked to death on December 13 and 14," the statement said.
The attacks took place in several villages near Beni, a town which lies about 150 miles (250 kilometres) north of the provincial capital Goma, MONUSCO said.
The UN peacekeeping force, one of the world's largest, has been tracking down armed groups in the region in recent months but did not provide details on who the attackers might be.
However, the North Kivu Civil Society umbrella organisation swiftly issued a statement charging that "the carnage was perpetrated by ADF-Nalu's Ugandan rebels".
