UN troops could not have prevented the rape of more than 150 women and boys by rebels in DR Congo because they did not know it was happening, a UN envoy said.
Peacekeepers passed through the area twice but were told only that rebels were setting up road blocks, he said.
UN Secretary General Ban Ki-moon said he was "outraged" by the attacks and has sent two envoys to investigate.
The UN has called an emergency session of the Security Council to discuss a response to the violence.
The rapes happened in Luvungi town and surrounding villages, within miles of a UN peacekeeping base, a US aid worker and a Congolese doctor have said.
Some reports say rebels occupied the area and gang-raped nearly 200 women and some baby boys over four days before leaving.
A UN joint human rights team confirmed allegations of the rape of at least 154 women by fighters from the Rwandan FDLR militia and Congolese Mai-Mai rebels in the village of Bunangiri.
But Roger Meece, a UN official in eastern DR Congo, said that while local people had told the UN patrols about roadblocks, they said nothing about the sexual violence. The UN was only told about it 10 days later by an aid group.
Speaking to journalists by video from Goma, Mr Meece said the villagers may have feared reprisals from the rebels or have been ashamed by the cultural stigma of rape.
But the BBC's Barbara Plett at the UN says there was clearly a serious failure in communications, made all the more significant as the peacekeepers work from a small forward operation base established to increase the UN's contact with civilians in the volatile region.
Mr Meece said the UN was now investigating ways of improving communication with local people.
One idea is for villagers to contact the base daily, "with the default being that if the communication is not made, there would be an assumption of a problem and a patrol despatched," he said.
'Must speak out'
DR Congo has a shocking reputation for sexual violence and rape is commonly used as a weapon of war.
But even by normal standards, the latest attacks were particularly vicious, says our correspondent.
Mr Ban said he had met victims of "appalling crimes of sexual violence" in DR Congo last year and felt compelled to ask whether more could have been done to protect the latest victims.
"Women and children should not have to live in fear of rape. Communities should not suffer the indignity of knowing that human rights abusers and war criminals can continue to behave with impunity," he said.
"We must speak up and we must act."
The UN has previously described Congo as "the rape capital of the world", with more than 8,000 women raped during fighting in 2009.
A report released in April by the Harvard Humanitarian Initiative showed that 60% of rape victims in South Kivu province had been gang-raped by armed men.
More than than half of the assaults took place in the victims' homes, the report said, and an increasing number of attacks were being carried out by civilians.
Eastern DR Congo is still plagued by army and militia violence despite the end of the country's five-year war in 2003.
UN peacekeeping troops have been backing efforts to defeat the FDLR, whose leaders are linked to the 1994 genocide in Rwanda and who are operating in eastern DR Congo.