Kasereka Ise Mighambo, the mayor of Lume in North Kivu province, said ADF fighters entered the town Thursday night and burned down a health clinic, killing four patients inside.
He added that the militants killed nine more civilians before retreating into Virunga National Park, a world-renowned wildlife haven in Congo's conflict-torn east.
Local nurse Kule Bwengue also said health authorities had registered 13 deaths in the attack.
The army killed three fighters and captured one when it responded to the attack , said army spokesman Anthony Mualushay.
"In the hospital ward there were four patients who all burned to death, in the pediatric unit all the mattresses are burned and in the side wards we just collected nine bodies," said Kule Mwenge Salomon, a nurse at Lume health center.
Kakule Vikere Lem was feeding his father at the clinic when he saw a column of people with torches approaching the town, around 40kms (25 miles) south-east of the city of Beni.
"I fled, thinking that they would spare the hospital, but unfortunately they burned my father in the hospital," Vikere said.
Claimed by the Islamic State group as its Central African offshoot, the ADF has been accused of massacring Congolese civilians and carrying out terrorist attacks in neighbouring Uganda.
The DRC and Uganda launched a joint offensive against the ADF in November 2021 to crush the rebels, but violence against civilians continues.
More than 120 armed groups roam eastern DRC.
This report was produced with information sourced from Agnece France-Presse and Reuters.