Government spokesman Lambert Mende said Monday that about 10 army soldiers have died in the fighting north of Goma, the provincial capital.
The United Nations mission in Congo says it started Sunday when the M23, which holds territory near Goma, attacked government troops near the towns of Mutaho and Rusayo.
Inhabitants of a village in the eastern DRC say they saw armed men in women's clothing enter the country from Rwanda on Sunday, shortly before fighting broke out near their village. The fighting continued Monday eight kilometers north of Goma, a city on the Congolese-Rwandan border.
This was the first heavy fighting at the front lines between government forces and the rebel group M23 since late May.
Many civilians in the rebel zone fled to the government side just before the fighting started. A farmer, Bifumbo Ruhira, said he left his home in the rebel zone when he saw more than 100 combatants arrive in his village early Sunday.
He said he saw many soldiers on board two trucks, and they were wearing women’s shawls over their uniforms. One of them told the villagers they were Rwandan soldiers and they had come from Rwanda as reinforcements.
The trucks stopped on the Rwandan side of the border, he said, and the combatants had run across.
Another villager, Baraki Murefu, confirmed his account, saying the combatants were wearing shawls as women wear them. It was a disguise so that people would not see they were soldiers, he said, and he noted they were heading to the northern side of Mutaho, where the M23 were positioned.
He said the fighting started shortly afterwards.
An M23 spokesman, Vianney Kazarama, denied the rebels had started the fighting and dismissed the story about armed men disguised as women.
"No, no, no," he said. "That is the Congolese culture. They like rumors."
He added that he does not know about the comings and goings at the frontier.
At the front, the M23 appears to be outgunned by the Congolese army.
That was the M23 firing, according to local journalists Sylvain Muyali and Mustapha Matabaro. Most of the mortar and rocket fire appeared to be coming from the government side
It is the Congolese army that is firing now, said Muyali.
If we go further, we will be in front of their position," said Matabaro. "At that point we turned back."
"Back on the main road, the Congolese army was moving up reinforcements. Three tanks and eight troop carriers and jeeps raced past us," said said Matabaro. "These were significant reinforcements for the army, and others were moving up through the day.
The M23 force is thought to be comprised of about 2,000 fighters.
Army colonel Jacky Zeng had a theory about why the M23 had launched the attack. "They do not want to be forgotten," he said. "They are reminding us they exist."
It may have been a costly reminder. The government spokesman Lambert Mende has said the rebels suffered about 120 casualties on Sunday, while government forces suffered six or seven.
Peace talks between the government and M23 in Kampala stalled again last week.
Local sources said the government troops advanced Monday to attack Kibati, the village in the M23 zone where the farmer Ruhira said he saw armed men in women’s clothing.