The United Nations is investigating claims that rebel fighters raped more than 150 women and baby boys in the Democratic Republic of Congo.
The attacks happened over four days within miles of a UN base, a US aid worker and a Congolese doctor said.
UN chief Ban Ki-moon is sending two top aides to the country to help investigate the alleged assaults in the country's volatile eastern region.
Mr Ban also urged the Congolese government to investigate the attacks.
Aid workers and UN representatives knew that rebels had occupied Luvungi town and surrounding villages in eastern DR Congo the day after the attack began on 30 July, the International Medical Corps (IMC) said on Tuesday.
They could not get into the town until the rebels left, said the IMC's Will Cragin.
According to reports, the rebels gang-raped nearly 200 women and some baby boys over four days before leaving.
The region lies approximately 10 miles (16km) from a UN peacekeepers' base.
Mr Ban is sending Atul Khare, assistant secretary-general for peacekeeping, immediately to DR Congo to help investigate, UN spokesman Martin Nesirky said.
He also ordered his special representative for sexual violence in conflict, Margot Wallstrom, to take charge of the UN's response to the attacks.
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, Mr Nesirky said.
"The secretary-general is outraged by the rape and assault. This is another grave example of both the level of sexual violence and the insecurity that continue to plague Congo," he told the Associated Press.
'World rape capital'
The victims are receiving medical and psychological care.
Ms Wallstrom condemned the rapes. She said: "It should be noted that this incident represents a very extreme case in terms of its scale and the level of organisation of the attacks.
The "terrible incident" confirmed her findings during a recent visit to Congo of the "widespread and systematic nature of rape and other human rights violations."
DR Congo has a shocking reputation for sexual violence. In April, a senior UN official said it was "the rape capital of the world".
A report 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.
More than 8,000 women were raped during fighting in 2009, the UN says.
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.