More than 30 people were killed and many more wounded when an armed group attacked villages in northwestern Central African Republic on Tuesday, the UN's peacekeeping mission (MINUSCA) said.
The massacres took place in several villages near the town of Paoua, not far from the Chad border, Major General Pascal Champion, head of MINUSCA's police component, said at a press conference in Bangui on Wednesday.
"Criminals arrived and opened fire" on the residents, he said.
Thirty-one people were killed in Koundjili and Lemouna. At least three other were killed in Bohong.
Local security forces also reported the deaths of 15 people in Maikolo village, but this information was not immediately confirmed by the UN.
The slaughter was the biggest single loss of life since the government and 14 militias signed a deal in February aimed at restoring peace to one of Africa's most troubled countries.
The UN peacekeeping mission "utterly condemns" the killings, MINUSCA head Mankeur Ndiaye said in a tweet on Wednesday.
"The authors of crimes such as these will be sought, arrested and brought to justice."
News of the bloodshed coincided with an announcement by the Vatican that a French-Spanish nun had been brutally murdered in a village in southwestern CAR.
In Rome, Vatican News said that the 77-year-old was found beheaded early Monday in the town of Nola, where she worked in a centre to help young girls.
"According to a local member of parliament, the murder could be linked to trafficking in human organs," the site said.
A UN source said the killings in the northwest were carried out by a group called 3R, which hosted a meeting with the villagers and then gunned them down indiscriminately.
The group takes its initials from three words in French meaning "Return, Reclamation and Reconciliation". It claims to represent the Fulani, one of the CAR's many ethnic groups.
The militia was one of the 14 groups that signed the February 6 peace accord, the eighth attempt to end violence in the impoverished country.
Under the deal, the government gave out key positions to warlords and agreed to set up mixed units of regular troops and militiamen.
The head of 3R, Bi Sidi Souleymane, also known as Sidiki, was appointed one of three "special military advisors" to the prime minister, in charge of setting up the combined units.
Communications Minister Ange-Maxime Kazagui, in a joint press conference with the UN, said "the government joins MINUSCA in demanding that Mr Sidiki arrest those responsible for this massacre within 72 hours, or else be held responsible for these acts."
"The government urges the public not to yield to the temptation of reprisals," Kazagui said.
Unconfirmed reports from the region say that so-called "anti-balaka" groups - militias claiming to protect Christians and animists from attacks by Muslim groups - are mobilising.
The CAR has been struggling to recover from the bloodletting that erupted when former president Francois Bozize, a Christian, was overthrown in 2013 by mainly Muslim Seleka rebels.
Armed groups, typically claiming to defend an ethnic or religious group, control about 80 percent of CAR, often fighting over access to the country's mineral wealth.
In a population of 4.5 million, thousands have lost their lives, nearly 650 000 have fled their homes and another 575 000 have left the country, according to UN figures as of December last year.
Pope Francis paid tribute to the murdered nun, describing her as a woman "who gave her life for Jesus in the service of the poor" and her killing as "barbaric".
He called for those gathered in St. Peter's Square for his weekly general audience to pray in silence for her.