The attack took place on Monday night on the outskirts of Oicha, a town in Beni territory, North Kivu province.
The IS-affiliated Allied Democratic Forces (ADF) are accused of slaughtering thousands of civilians in the conflict-torn eastern DRC.
"So far we have 20 bodies... tension is high, the ADF have once again plunged Oicha into mourning," mayor Nicolas Kikuku told AFP by phone on Tuesday.
"We have just placed 26 bodies in the morgue of Oicha's general hospital," said Darius Syaira, Beni territory's civil society representative.
He said the victims were 12 minors and 14 adults, most of them killed with knives.
Syaira also said tensions were high in Oicha, with demonstrators setting fire to humanitarian vehicles preparing to distribute food.
"We don't need humanitarian aid, but we do want security," one of the demonstrators said when asked why residents had attacked the trucks.
Originally made up of mainly Muslim Ugandan rebels, the ADF gained a foothold in eastern DR Congo in the 1990s.
Since 2019, some ADF attacks in eastern DR Congo have been claimed by the Islamic State group, which calls the fighters the Islamic State Central Africa Province.
Police said the ADF were responsible for killing a honeymooning couple and their safari guide in one of Uganda's famed national parks on October 17. The Islamic State group claimed responsibility for the attack.
Uganda and the DRC launched a joint offensive in 2021 against the ADF to drive the militants out of their Congolese strongholds, but attacks have continued.