Some 240 women, girls and babies may have been raped after rebels recently seized a town in the Democratic Republic of Congo, the UN says.
Officials had previously said they had received reports of 150 rapes in and around the town of Luvungi.
The UN mission has been heavily criticised for not doing more to protect the local population as it had peacekeepers based nearby.
But it says it was only told of the rapes after the rebels had left.
The incident prompted an emergency session of the UN Security Council, and Secretary General Ban Ki-moon has sent a senior envoy to find out what happened after rebels moved into Luvungi on 30 July, staying for four days.
The council also said peacekeepers in the area should have done more to protect local people from the Congolese Mai Mai and Rwandan FDLR armed groups.
The FDLR has, however, denied that its forces took part in the attack.
The peacekeepers say they were not told about the attacks until 10 days later, even though they have a base 20 miles (30km) away.
They say local people may have been afraid of rebel reprisals or ashamed by the rapes.
The UN mission in DR Congo, known as Monusco - until recently the world's largest peacekeeping mission - says it has stepped up patrols in the area "to reassure the local population", reports the AFP news agency.
Some of the women report being abused by several men in front of their husbands and children.
The DR Congo conflict has become notorious for the sexual abuse of women and girls - one UN envoy called it the "rape capital of the world" earlier this year.
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.