The claim comes 10 years after Rwandan Hutu extremists massacred their country's Tutsi minority.
Dissident General Laurent Nkunda was the first to make accusations of ethnic massacres in eastern DRC recently.
Nkunda on 2 June led a group of soldiers into the eastern town of Bukavu, near the border with Rwanda, saying he was seizing the provincial capital to protect his fellow Banyamulenge, DRC ethnic Tutsis with historic ties to Rwanda.
A week after seizing the town, Nkunda withdrew his accusations and his men.
But on Sunday, he indirectly revived the claims of massacres when he threatened Kinshasa with war if it did not set up a commission to investigate the alleged ethnic slaughter in Bukavu.
The DRC army on Monday denied that its soldiers had taken part in massacres in Bukavu, calling the accusations "lies and excuses" for Nkunda to plunge the country into another war.
And on Wednesday, the head of the human rights branch of the UN mission in DRC, MONUC, said after a visit to Bukavu that there had been "no planned massacre" in the town.
"Massacres did indeed take place but you cannot say that a particular community was targeted"
head, human rights branch, MONUC, Congo
"Massacres did indeed take place but you cannot say that a particular community was targeted," said Roberto Ricci.
Nkunda and other dissident officers "have imported ideas that are alien to the Congolese culture ...by talking about things that have no place in Congo, if you ask me," he said.
Meanwhile, a key Banyamulenge leader, Benoit Mubanda Kadage, has jumped on Nkunda's bandwagon.
Mubanda, who is head of the Banyamulenge community in the eastern towns of Bukavu, Uvira, Goma, and Minembwe, the capital, Kinshasa, and the ethnic group's diaspora, said in a statement published on Wednesday that events in Bukavu "constitute acts of genocide."
But transition lawmaker, Enoch Ruberangabo Sebineza, also a member of the Banyamulenge community, has taken an entirely different tack.
He denounced Nkunda and the other dissident soldiers in eastern DRC as "true criminals, whom the Banyamulenge community does not need."