Members of the Patriotic Alliance for a Free and Independent Congo (APCLS) militia said combat was ongoing in the Ruhanga and Kihonga areas of North Kivu province's Rutshuru region.
"We are in the middle of fighting," said a APCLS commander.
APCLS spokesman Heritier Ndangendange also confirmed that clashes had erupted with the M23.
The armed group claims to represent the Hunde ethnic group, and is dedicated to thwarting the M23, which has captured swaths of territory in North Kivu in recent months.
A Tutsi-led rebel group, the M23 is one of scores of militias active in the Democratic Republic of Congo's turbulent east, many of them legacies of two brutal wars that flared at the turn of the century.
It briefly occupied North Kivu's capital Goma in 2012 before being beaten back and going to ground the following year.
But M23 fighters took up arms again in late 2021, blaming the Congolese government for failing to honour a commitment to integrate its fighters into the army, among other grievances.
They have since advanced across Rutshuru -- winning a string of victories against the Congolese army -- and come within several dozen kilometres (miles) of Goma, a commercial hub of over a million people.
The APCLS and its allies from the pro-Hutu Nyatura group also clashed with M23 rebels on December 16.
Congo accuses its smaller central African neighbour Rwanda of backing the M23, something which Rwanda has repeatedly denied.
But US and French officials, as well as United Nations experts, agree with the assessment.