"The victims were taken hostage Tuesday during an ADF raid" at Babila Babombi, a locality in Mambasa, a territory regularly targeted by ADF and other militia groups, police administrator Matadi Muyapandi told AFP.
"There are 20 dead -- 16 men and four women," he said, adding four people who escaped had been hospitalised.
"The victims had been decapitated, the modus operandi" of the ADF, originally mainly Muslim Ugandan rebels, who have established a presence over the past three decades in eastern Democratic Republic of Congo, killing thousands of civilians.
Recent months have seen a slew of such attacks.
The victims were captured in several attacks on surrounding villages, then "grouped in the forest to be executed", said Rams Malikidogo, a human rights defender in Mambasa.
"The victims are gold miners, traders, farmers," a local aid worker reached by phone told AFP on condition of anonymity.
The killings were perpetrated in an isolated place and "we need the presence of the military to recover the bodies and bury them with dignity," he added.
The ADF pledged allegiance in 2019 to the Islamic State group, which portrays them as its central African branch.
The ADF has been accused of massacring Congolese civilians as well as staging attacks in neighbouring Uganda.
Since the end of 2021, the Congolese and Ugandan armies have conducted joint operations against the ADF in North Kivu and the neighbouring province of Ituri but have so far failed to stop the deadly attacks on civilians.
Some experts believe, however, that the operations have only dispersed the rebels to areas difficult to access where they continue to target civilians.