The officials on Wednesday told the Security Council that defeating the Democratic Forces for the Liberation of Rwanda, or FDLR, will be difficult because unlike the M23 they live among the general population, increasing the risk of civilian deaths. But the officials sounded more optimistic about the troubled region than they have in a long time. "Many areas are liberated," said the U.N. Special Representative in Congo, Martin Kobler. You see it in the eyes of the population, and there is still some hesitancy, but there is a chance that this time the situation is irreversible. A document formalizing the end of hostilities between the Congo government and the M23 will be signed Thursday in Nairobi, said U.N. Special Envoy to the Great Lakes Region Mary Robinson. Negotiations took about a year. The offensive against the FDLR began Nov. 27. Other, smaller armed groups also are the target, but some already are giving up their weapons with the defeat of the M23, which was widely believed to be financed and backed by Rwanda. In Kinshasa, the commander of the U.N. peacekeepers in Congo, Lt. Gen. Carlos dos Santos Cruz, said the FDLR "has one last opportunity to turn themselves in, and if not, we will remove them." Congo, with a population of 66 million, has been ripped by conflict for nearly two decades. Armed groups have thrived in its mineral-rich forests, despite the presence of nearly 20,000 blue-helmeted U.N. peacekeepers. The Security Council gave approval in January for the trial use of unarmed drones for intelligence gathering there. Associated Press writer Rukmini Callimachi contributed.