The top UN envoy in Congo said two peacekeeping patrols were not informed by villagers that mass rapes were taking place and the United Nations is now working to improve communications and prevent any recurrence.
Roger Meece, the new UN special representative, said peacekeepers did not learn about the "horrific" rapes of at least 154 Congolese civilians for nearly two weeks, which showed that the force's actions to protect civilians were insufficient and need to be improved.
He said one idea being pursued was to have villages report to the UN's forward operating base at Kibua every day.
If the force did not receive a report, he said, it would assume there was a problem and send a patrol to investigate.
Mr Meece gave the most detailed account of the UN's actions since Monday's report that Rwandan and Congolese rebels gang-raped nearly 200 women and some baby boys over four days not far from Kibua in eastern Congo's mining district.
He spoke to reporters at UN headquarters by videoconference from Goma in eastern Congo.
Will F. Cragin of the International Medical Corps said on Monday that aid and UN workers knew rebels had occupied Luvungi town and surrounding villages the day after the attack began on July 30.
He said his organisation was only able to get into the town after rebels ended their brutal spree of raping and looting and withdrew of their own accord on August 4.
The UN was not made aware of the attacks until more than a week later, despite the fact that UN patrols had been in Luvungi twice after the attacks began.
Pressed on why two UN patrols learned nothing about the mass rapes, Mr Meece said he could only speculate, noting that communication is always a problem in Congo.
"There is, of course, a significant amount of cultural baggage ... associated with rapes in this area, as well as elsewhere." he said.
"Is it conceivable that the local villagers were afraid of reprisals if they reported anything to Monusco? Possible. Is it conceivable that they were ashamed of what has happened in some form? That's possible.
"I can only speculate as to what may have been the reasons, but I know that these can be very powerful in the local society and environment," he said.
According to an American aid worker and a Congolese doctor, the rebels gang-raped nearly 200 women and some baby boys.