Elements of the ADF, an Islamic State-affiliated insurgent group that originated in neighbouring Uganda, carried out the attack in Komanda town, Irumu territory, Ituri province.
At least 49 civilians were killed, including nine children. Several other people were injured and abducted, and shops and homes were set on fire.
Most of the victims were reportedly worshippers killed with bladed weapons during a night vigil at a church.
The incident comes after ADF attacks earlier in the month killed 82 civilians in Ituri and North Kivu provinces, which MONUSCO condemned on 23 July.
The group has committed grave human rights violations against civilians since its founding in 1995 and has been under UN sanctions since June 2014.
MONUSCO expressed “deep outrage at these heinous acts of violence, which constitute serious violations of international humanitarian law and infringements on human rights”.
The peacekeeping mission also extended condolences to the bereaved families, urged Congolese authorities to investigate these killings, and reiterated the UN Secretary-General’s call for all foreign armed groups to lay down their weapons unconditionally and return to their countries of origin.
“These targeted attacks against defenceless civilians, particularly in places of worship, are not only appalling, but also in violation of all human rights standards and international humanitarian law,” said Vivian van de Perre, Deputy Special Representative of the UN Secretary-General for Protection and Operations and Acting Head of MONUSCO.
She added that the mission “will continue to work tirelessly alongside the Congolese authorities to protect the population in line with its mandate”.
MONUSCO is supporting local authorities in their response by organising burials and providing medical care to the wounded.
The peacekeeping mission is also intensifying security efforts in and around the city of Komanda.
The mission is committed to working with Congolese authorities and communities “to help prevent future attacks, protect civilians, reduce tensions and contribute to the stabilization of areas affected by armed violence”.