The Democratic Republic of Congo's army on Thursday accused M23 insurgents of killing 50 civilians and breaching a five-day-old truce in the country's restive east.
The rebel group issued a statement late Thursday denying the alleged massacre of civilians.
The ceasefire took effect in North Kivu province at the weekend following a summit between DRC and its neighbour Rwanda.
It was to have been followed by a rebel pull-out from captured territory, a withdrawal that has yet to take place.
General Sylvain Ekenge said the M23 group was "carrying out massacres... the most recent of which is that of 50 Congolese civilians, heinously murdered on Tuesday in Kishishe," a village some 70 kilometres north of the eastern city of Goma.
Ekenge claimed that while Congolese forces had "scrupulously observed the truce", the M23 had attacked government positions.
The M23 responded with a statement describing accusations of a massacre in Kishishe as "baseless allegations" and insisting that "it has never targeted civilian populations".
Sources said earlier that fighting had resumed Thursday in Kirima in the same region, about 10 kilometres from the town of Kibirizi.
"The rebels have crossed the bridge, heading for Kibirizi... there's panic," said Paul Lutibahwa, head of civil society groups for the Bambo region.
A security official, speaking on condition of anonymity, accused the M23 of having breached the ceasefire and "carrying on looting and fighting".
"The fighting is heavy — we are using heavy artillery," said a DRC army officer who also asked not to be identified.
Contacted by AFP, M23's military spokesman Willy Ngoma confirmed that there was fighting with the army.
The March 23 movement, or M23, is a predominantly Congolese Tutsi rebel group that was dormant for years.
It took up arms again in November last year and seized the town of Bunagana on the border with Uganda in June.
After a brief period of calm, it went on the offensive again in October 2022, greatly extending the territory under its control and advancing towards the city of Goma.
Kinshasa accuses its smaller neighbour Rwanda of providing M23 with support, something that UN experts and US officials have also pointed to in recent months.
Kigali disputes the charge, and in turn accuses Kinshasa of collusion with the FDLR — a former Rwandan Hutu rebel group established in the DRC after the 1994 genocide in Rwanda.
Talks between the two countries in the Angolan capital of Luanda unlocked a truce agreement on November 23.
The ceasefire was scheduled to take effect on Friday, November 25 at 1600 GMT and be followed by a pull-out by the M23 two days later.
A parallel initiative has been undertaken by the East African Community (EAC), a seven-nation regional bloc that includes Rwanda.
It has decided to deploy a regional force to help stabilise the region, for which Kenyan troops are already deployed in Goma, and on November 28 launched peace talks, to which the M23 are not invited.
Until Thursday's violence, there had been no fighting between government forces and the M23, although the rebels had clashed with local militia, especially in the Bambo area, where civilian casualties were reported.