Fighters from the Cooperative for the Development of the Congo (CODECO) militia group, which is made up of fighters from the Lendu ethnic group, attacked the Hema village of Ndjala in Ituri province at around 1 a.m. on Sunday, the army and local authorities said.
"They cut with the machetes several of my compatriots, 20 have already died and more than 14 seriously injured," said Solo Bukutupa, a local administrator.
"It's unbearable to see people die like that."
The attackers fled after United Nations peacekeepers arrived at the village and they later fired on a nearby UN base, a UN source said.
Fighting by an array of armed groups in the region has complicated Congo's response to the coronavirus pandemic and an Ebola epidemic that has killed more than 2,200 people since 2018.
CODECO split into several competing factions after the Congolese army killed its leader Justin Ngudjolo in late March.
Earlier this month Ngabu Ngawi Olivier, who claimed to have taken over the leadership of CODECO, surrendered to the army and called for the militia to lay down its weapons.
Another faction later issued a statement denouncing Olivier as an imposter. No fighters have followed Olivier's orders yet, said army spokesman Jules Ngongo.
Rich in natural resources including gold, diamonds and coltan, Ituri province was the site of some of the country's worst fighting between 1999 and 2007, after a power struggle between rebel groups descended into ethnic violence -- much of it between the Hema and Lendu.
After several years of relative calm, tit-for-tat fighting erupted again in December 2017, reviving longstanding tensions over land.
The unrest has since evolved into more coordinated attacks by the Lendu community on the army and the Hema ethnic group.
Late last year the army launched a large-scale operation to uproot a constellation of militias operating in the east of the country, sparking a backlash that has seen at least 350 people killed by armed groups in Ituri, the UN source said.