Meanwhile, the UN said it was investigating reports that rebels led by Gen Nkunda had killed civilians in their homes in the eastern town of Kiwanja on Thursday.
At least 12 bodies were found in the town, which was retaken by rebels loyal to him earlier this week.
"They knocked on the doors, when the people opened, they killed them with their guns," said Simo Bramporiki, whose wife and child were killed during the night.
One woman showed journalists the bodies of five men inside her house, one of them her husband, and two more lying outside.
Gen Nkunda claimed his fighters, who have also seized control of the town of Nyanzale, about 80km (50 miles) north-west of Goma, had attacked armed pro-government militia.
However, reports say there was nothing to indicate the men, most of whom were wearing civilian clothing, were fighters.
A spokesman for the UN in DR Congo, Madnodje Mounoubai, told the BBC that the UN was doing its best to help civilians, but that peacekeepers could not fire at rebels when they were surrounded by civilians.
"You cannot fire when you have civilians on the road running in all directions. If you start firing in that situation you end up killing a lot of civilians," he said.
The situation in DR Congo has been described as a humanitarian catastrophe, and an estimated 250,000 people have been made homeless by the conflict.
The UN says it is investigating the killings in Kiwanja
The UN has 17,000 peacekeepers in DR Congo, making the mission, Monuc, its largest in the world.
But only a few hundred peacekeepers are in the areas affected by the latest violence, and human rights groups have also criticised the UN for failing to prevent the killings.
However, Monuc says it is reinforcing its troops in the regional capital, Goma, and has warned that soldiers will fire on any armed group trying to enter the city.
Meanwhile, the charity Save the Children says there has been an increase in the kidnapping of children to become child soldiers.
Before the fighting began in earlier this year, there were an estimated 3,000 child soldiers across the country, but that number is now thought to be much higher.
"At the beginning of June we were thinking that in this region there were about 3,000 child soldiers... The numbers now, after this latest upsurge of recruitment and violence, are likely to be very much higher," said spokeswoman Ishbel Matheson.
Gen Nkunda has threatened to topple the DR Congo government in Kinshasa, 1,580km (980 miles) west of Goma, unless President Kabila agrees to hold direct talks.
The Congolese government has refused to negotiate with Gen Nkunda, calling him a terrorist.
He says he is fighting to protect his Tutsi community from attack by Rwandan Hutu rebels, who fled to Congo after the 1994 genocide.
Gen Nkunda defends his military actions
The Congolese government has often promised to stop Hutu forces from using its territory, but has not done so.
For its part, the Congolese government accuses Rwanda trying to destabilise it of by backing the rebels, which Rwanda denies.
Gen Nkunda also objects to government plans for foreign involvement in exploiting the country's vast mineral wealth.
Correspondents say a ceasefire around Goma appears to be holding, but the latest clashes have sparked fears the rebels could follow through on their threat to attack the city.
Tens of thousands of people have sought refuge there.
Rwandan President Paul Kagame has blamed the international community for failing to address what he says are the root causes of the crisis, despite channelling tens of millions of dollars into the region.
He said the real reason for the crisis was what he described as the weak leadership of the Congolese government, which had failed to disarm Hutu rebels.
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