Rwandan Hutu fighters in the Democratic Republic of Congo could jeopardise the new peace pact, DR Congo's UN ambassador Atoki Ileka has admitted.
Rwanda denies backing General Laurent Nkunda
Renegade rebel Gen Laurent Nkunda, who says he has been fighting to protect DR Congo's Tutsi community, signed the agreement, dependent on them disarming.
They have lived in eastern DR Congo since the Rwandan genocide in 1994.
But a Hutu rebel representative told the BBC the group would not leave DR Congo until Rwanda agreed to negotiate.
Neither the Rwandan government nor representatives of the Democratic Front for the Liberation of Rwanda (FDLR) - more commonly referred to as the Interahamwe - took part in the talks where the peace deal was negotiated.
The Tutsi-dominated Rwandan government has twice invaded DR Congo, saying it is trying to stop Hutu rebels from attacking its territory.
Gen Nkunda has always denied charges that he is working for Rwanda.
The agreement, which involved about 20 armed groups, aims to end months of bloody conflict in eastern DR Congo which has driven almost 500,000 people from their homes.
The head of the UN mission in DR Congo, who was present at Wednesday's signing ceremony, said he was optimistic about the future, although there would be difficulties ahead.
Alan Doss told the BBC the peacekeeping force should be adequate to police the settlement.
Mr Ileka told the BBC the authorities hoped to be able to disarm Hutu militias "by peaceful means."
"But if not we will have to do something about it," he said.
Reacting to news of the deal, the FDLR's Callixte Mbarushimana asked for patience from the Congolese people.
"Should the government of Rwanda sit down with us at the negotiating table we are going to stop everything," he told the BBC from Paris.
Africa analyst Muzong Kodi at Chatham House in London says the exclusion of Rwandan elements is a real failing of the agreement.
"Without Rwanda deciding to talk to the FDLR and finding a political solution to the problem they've had since 1994, there's just no way the war is going to end," Mr Kodi said.
But Michel Mungunda of the aid agency Cafod said that if it held, the peace agreement could benefit huge numbers of people who had suffered years of conflict.
"It will make a very, very big difference, especially for the internally displaced people, who for a long time have been living in camps," he said.
The International Rescue Committee says the death toll in the past decade in DR Congo has surpassed any conflict since World War II - 5.4m.