Rwandan Hutu rebels in eastern Democratic Republic of Congo say a plan announced on Saturday to disarm them by force will not work.
The FDLR want security guarantees before returning to Rwanda
DR Congo and Rwanda agreed a deal to forcibly disarm the Democratic Forces for the Liberation of Rwanda (FDLR) and urged sanctions against armed militias.
Insurgents led by Gen Laurent Nkunda say they are fighting to protect Congolese Tutsis from the FDLR.
Some 370,000 people have fled the fighting since the start of this year.
Aid agencies say there is overwhelming evidence of atrocities by fighters from all sides against the civilian population, including mass rape of women and children.
After talks in the Kenyan capital, Nairobi, a joint statement said the DR Congo would "launch military operations, as a matter of urgency, to dismantle the ex-FAR/Interhamwe" which it called "a genocidal military organisation in the DR Congo".
Rwanda and the DR Congo also urged the UN Security Council to impose sanctions against armed groups in DR Congo.
Reacting to news of the deal, Calicte Mbarushimana of the FDLR, told the BBC's Focus on Africa programme the government "can do whatever they want as a sovereign government".
"But it is not a good solution to go for war again, it is better to solve the problem through dialogue," he said.
Saturday's statement said that FDLR rebels, who were not participants in the 1994 Rwandan genocide and who agree to disarm, could be allowed to opt for Congolese nationality and remain in the DR Congo, but in areas far from the Rwandan border.
Rwanda's Foreign Minister, Charles Murigande, told the BBC that there was also a provision in the agreement for those Hutus who wanted to return to Rwanda, to do so.
"This group is clearly the biggest security threat to the region - if they were to be completely disarmed, the region would definitely make a quantum leap towards peace, security and stability," Mr Murigande said.