Congolese troops and M23 rebels bombarded each other near the Ugandan border on Monday and international envoys called on both sides to cease fire and allow a peace deal to take hold.
Congo's army accused the rebels of shelling the frontier town of Bunagana and said it showed M23's ceasefire declaration at the weekend was worthless.
The rebels said they were ready to sign a peace deal but that they had been attacked with heavy weapons.
A rapid army advance in the last few weeks has driven rebels from towns and cornered them in the steep, forested hills along the Ugandan border, raising the prospect of an end to a 20-month rebellion that has gripped Congo's mineral-rich east.
South Africa is hosting leaders from the Great Lakes region and southern Africa to back Ugandan-sponsored peace efforts, but the latest violence shows the Kinshasa government and the rebels remain far apart.
"This is not fighting, it is bombs launched by M23 targeting the population of Bunagana," Congo army spokesman Colonel Olivier Hamuli said by phone. "They are targeting civilians."
Bunagana was the last rebel-controlled town to be recaptured last week and rebels are still in the hills nearby. A Reuters reporter said government troops had seized Mbuzi, a strategic hilltop above Bunagana, on Monday after hitting it with tank and rocket fire. Seven rebels were captured, he said.
On Sunday M23 announced a ceasefire - a declaration Hamuli described as a lie, saying the army would pursue the rebels.
Citing Congolese military sources, United Nations-backed Radio Okapi said on its website that four civilians had been killed and 10 wounded in Bunagana on Monday. Reuters