On Tuesday, rebels and troops were still clashing despite a ceasefire, brokered by Angola last week month, that was due to start at noon.
On March 3, the Angolan presidency - which is mediating the conflict - announced a ceasefire for eastern DRC taking effect on Tuesday.
However, "The fighting continues," said a security official in the Democratic Republic of Congo, who declined to be named.
M23 spokesman Willy Ngoma told AFP on Tuesday that the group was defending itself after army attacked all of the group's positions simultaneously.
Lieutenant-Colonel Guillaume Ndjike, the army's spokesman in North Kivu, said late Monday the M23 "and their sponsors from the Rwanda Defense Force" had attacked a recently deployed Burundian contingent of an East African regional military force.
According to aid workers, fighting had erupted on Monday in North Kivu province, where the Tutsi-led M23 has captured swathes of territory since re-emerging from dormancy in late 2021.
Both sides accuse the other of triggering the fresh round of clashes.
The DRC accuses Rwanda of backing the M23 rebels, who have displaced over half a million people and are edging closer to the city of Goma, a trade hub.
Independent UN experts, the United States, as well as several other western states, agree with the assessment, although Rwanda denies it.
The seven-nation East African Community (EAC) decided to create a military force to stabilize eastern Congo last year, with the first troops arriving in Goma in November.