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 (40 miles) 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.
Paul Lutibahwa, head of civil society groups for the Bambo region, said: "The rebels have crossed the bridge, heading for Kibirizi... there's panic".
A security official, speaking on condition of anonymity, accused the M23 of having breached the ceasefire and "carrying on looting and fighting".
A DRC army officer, who also asked not to be identified, said: "The fighting is heavy -- we are using heavy artillery."
The M23's military spokesman, Willy Ngoma, contacted by AFP, 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, 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 genocide of the Tutsi community in 1994 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.
The Vatican, meanwhile, announced that Pope Francis would visit the DRC and South Sudan from January 31 to February 5.
His trip to the two violence-plagued countries had been planned for July this year but was postponed because of treatment for knee pain.
His stay in the DRC from January 31 to February 3 will take place in Kinshasa and no longer include Goma.
The site that had been chosen for a papal mass, located 15 kilometres (nine miles) north of Goma, is currently occupied by a forward position of the armed forces.
Scores of armed groups roam eastern DRC, making it one of Africa's most violent regions.
Many are legacies of two wars before the turn of the century that sucked in countries from the region and left millions dead.
Demonstrators protesting perceived international indifference to the crisis rallied in Goma early Thursday. Another march staged by the Catholic church took place in Bukavu, in neighbouring South Kivu province.