Villagers said the rebels killed civilians in their homes
The rebel forces of Gen Laurent Nkunda and pro-government militias in the Democratic Republic of Congo have been accused of war crimes by the UN.
The alleged crimes took place in the eastern town of Kiwanja this week when it was captured by Gen Nkunda's forces. Several civilians were reported killed.
The UN said investigators did not yet have a clear idea of what happened.
Fighting between government and rebels forces has left hundreds of thousands of people displaced since August.
The head of the UN mission (Monuc), Alan Doss, said he was concerned about what he described as "targeted killings of civilians by different armed groups" in Kiwanja, 80km (50 miles) north of the provincial capital Goma.
"We condemn them, we deplore them, and we remind the different groups involved that international law is very clear on this - these are war crimes that we cannot tolerate," he said, quoted by AFP news agency.
The UN investigation followed reports by Human Rights Watch that civilians were killed in their homes in Kiwanja, both as pro-government militiamen tried to take the town and after their departure.
At least 26 people are known to have died. UN military spokesman Jean-Paul Dietrich told the BBC that while some may have died in crossfire, others were summarily shot.
The rebels said they had attacked armed, pro-government militiamen, but reports say there was no evidence to suggest the dead people were fighters.
The investigation came as African leaders meeting in Nairobi called for an immediate ceasefire, for UN peacekeepers to be given a wider mandate and for humanitarian corridors to be set up to help the displaced.
The DR Congo government has accused UN peacekeepers of failing to stop rebels from killing civilians.
But while Rwanda has been accused of supporting the rebels a Uruguayan officer serving with the UN peacekeepers in DR Congo said the government side was being reinforced by Angolan troops.
The officer spoke to international news agencies in Goma, saying the Angolans had arrived there four days ago.
Two eye-witnesses have told the BBC they have seen Angolan troops in the area.
Angola is an ally of the Congolese government and has been invited by Mr Kabila to provide military assistance. But its government has said it will not intervene directly.
Angola and Zimbabwe both supported DR Congo with troops during the 1998-2003 war.
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 has 17,000 peacekeepers in DR Congo, making Monuc its largest mission 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.
Gen Nkunda says he is fighting to protect his Tutsi community from attack by Rwandan Hutu rebels, who fled to Congo after the 1994 genocide.