* Most hacked to death, some raped and beheaded
* Local officials blames Ugandan Islamist rebels
* U.N.-backed Congolese forces seeking to stamp out armed groups (Adds local official's quote, details)
By Pete Jones
KINSHASA, Dec 16 (Reuters) - At least 21 people, including women and a baby, were killed over the weekend in eastern Democratic Republic of Congo, the U.N. peacekeeping mission there said on Monday.
The U.N. mission did not say who it believed carried out the massacre but a local elected official blamed Ugandan Islamist rebels.
Most of those killed appeared to have been hacked to death on Friday and Saturday in villages not far from Beni, in Congo's North Kivu province, according to a statement by Martin Kobler, head of the MONUSCO peacekeeping mission.
"These atrocities will not go unpunished," he said. Three girls appear to have been raped and then beheaded while one of the victims was just months old, his statement added.
The killings highlighted the challenges still facing Congo's army and U.N. forces attempting to pacify Congo's mineral-rich east, which has remained a lawless patchwork of rebel strongholds and militia fiefdoms for nearly two decades.
U.N.-backed government forces won a rare victory last month against the Tutsi-dominated M23 rebels, the region's most serious uprising in years.
But dozens of armed groups still hold ground in the rugged mountains along Congo's eastern border with Rwanda and Uganda.
Jaribu Muliwavyo, a member of North Kivu's provincial assembly from Beni, told Reuters he believed the Allied Democratic Forces, a rebel group that emerged in the 1990s in opposition to the Ugandan government, were behind the attack.
The Ugandan government says the group, also known by the acronym ADF-NALU, is allied to elements of Somalia's al Qaeda-linked al Shabaab movement.
"ADF-NALU has been moving for the last two or three weeks and this locality was in their route. ADF-NALU controls this area," Muliwavyo told Reuters by telephone. "It is terrorism, pure and simple."
While they were largely defeated and driven out of Uganda in the mid-2000s, the ADF has held out in Congo.
The ADF now has up to 1,400 fighters and has kidnapped about 300 Congolese civilians over the past year, according to a report prepared by a new U.N. Intervention Brigade charged with helping Congolese forces hunt down armed groups.
Following the defeat of M23, the 3,000-strong U.N. Brigade launched operations earlier this month against the Rwandan FDLR rebellion, which includes Hutu militia members who helped carry out Rwanda's 1994 genocide. (Additional reporting by David Lewis; Writing by Joe Bavier; Editing by Andrew Heavens)