Some 113 rebels died in clashes on Thursday between the M23 rebel group and Democratic Republic of Congo troops, the regional governor said, as violence flared days after the UN and US imposed sanctions on the group's leader.
The fighting early Thursday near the eastern city of Goma came a day after the UN said armed groups in the region slaughtered over 200 people including scores of children between April and September.
Julien Paluku, governor of the resource-rich North Kivu province whose capital is Goma, added that "a few" members of the DR Congo government forces (FARDC) were wounded in the clashes, along with the 113 rebels killed, up sharply from a previous toll.
Government spokesman Lambert Mende had said earlier that "51 bodies (of rebels) wearing Rwandan army uniforms have been collected".
Mende added that three rebels were wounded, a captain of the group was caught and that artillery, including rocket launches, were found.
According to the army's spokesman for North Kivu, Lieutenant Colonel Olivier Hamuli, one FARDC commander died in the clashes.
M23 military spokesman Lieutenant Colonel Vianney Kazarama for his part refused to release a toll.
The M23, which has dubbed its armed wing the Congolese Revolutionary Army, was launched by former fighters in an ethnic Tutsi rebel group that was integrated into the Congolese military under a 2009 peace deal whose terms the mutineers claim were never fully implemented.
The United Nations has accused neighbouring Rwanda and Uganda of backing the rebels, but both countries deny this.
Rights groups accuse M23 of human rights abuses and of unleashing a fresh cycle of violence by the region's complex web of armed groups.
Its leader, Sultani Makenga, a former colonel in the DR Congo army, is accused of masterminding killings, sex attacks and abductions and recruiting child soldiers.
A rebel statement accused the DR Congo army of launching several offensives against M23 positions in the Rugari area, about 30 kilometres (20 miles) from Goma not far from the Rwandan border.
It said the fighting was a breach of an already shaky ceasefire.
"The FARDC advanced to attack us (...) we must defend ourselves," Kazarama told AFP Thursday morning. He said the rebels would counter-attack "in self-defence".
Hamuli rejected this: "We did not attack them," he told AFP. "We know that they have been reinforcing their positions for more than two weeks."
Hamuli said fighting stopped in the afternoon but that the army was keeping up a search in the area. However, the M23 spokesman insisted that "the enemy continues to bomb our positions".
Hamuli said Thursday "a small group attacked us from Rwanda" as the Congolese army was involved in a separate push targeting rebels between Rugari and Kibumba which borders Rwanda.
Asked whether he could identify the small group he said: "How are we supposed to know who's who if the M23 and the Rwandan army are wearing the same uniform?"
The fighting came a day after the government in Kinshasa dismissed as inadequate UN and US sanctions against Makenga and said neighbouring Rwanda and Uganda should also be targeted.
Washington announced sanctions against Makenga on Tuesday, freezing his assets in US jurisdiction and forbidding any US citizen from doing business with him, accusing him of attacks on civilians and the recruitment of child soldiers.