Paul Kagame, the Rwandan president, has criticised world powers for "abandoning" the country to genocide in 1994, in which more than one million people were slaughtered.
The president made his comments to nearly 20,000 people on Tuesday at a ceremony marking the the 15th anniversary of the mass killings.
Kagame said of those who commanded the UN forces at the time: "They left them to be murdered. Aren't they guilty?
"We are not like those who abandoned people they had come to protect."
The ceremony was held in Nyanza, a site where thousands of people were killed four days after an attack on UN Belgian forces in the town led those troops to withdraw.
"They left even before any shot was fired. We are not cowards. They [world powers] are part of that history and the root causes of the genocide," he said.
Kagame laid a wreath as part of the ceremony and lit a torch in memory of the genocide's victims, mainly minority Tutsis and moderate Hutus killed by Hutu fighters known as the Interahamwe.
In depth
The ceremony also involved the reburial of some of the remains of those massacred in Nyanza.
Al Jazeera's Yvonne Ndege, reporting from Nyanza, said: "Many of them [those who fled to Nyanza] had come here to seek refuge and protection from a small UN mission that was based here.
"Of course, the UN eventually pulled out and at least 5,000 people met their deaths in a very brutal manner."
Benoit Kaboyi, the executive secretary of Ibuka, a genocide survivors' organisation, said: "Nyanza marks the failure of the international community. It is the failure of humanity as a whole."
Kigali has repeatedly accused world powers of not doing enough to hunt down the perpetrators of the genocide, many of whom remain at large in countries such as France, Belgium, Canada, Kenya and the Democratic Republic of Congo.
Rwanda has also criticised nations, particularly France, of failing to acknowledge their role in allowing the killings.
Latest figures released by the UN and Rwanda's government say that at least 1.2 million people were killed in the genocide.
The beginning of the genocide was signalled in April 1994, when a plane carrying Juvenal Habyarimana, the former Rwandan president, and Cyprien Ntaryamira, his Burundian counterpart, was shot down.
The killings of minority Tutsis and moderate Hutus by Interahamwe fighters continued over the next 100 days.
The genocide remains a source of tension, with some survivors saying they are targets for intimidation by their former foes and Hutus complaining of marginalisation by Kagame's government.
Rwanda's criminal investigations department has said that 2,178 cases of murder, torture and intimidation of genocide survivors had been recorded since 2007.