The M23 rebels, widely believed to be backed by Rwanda, captured the eastern city of Goma on Tuesday as United Nations peacekeepers looked on after Congolese troops had quit the town.
"The journey to liberate Congo has started now," said Vianney Kazarama, spokesman for the rebels.
Hours later, a rebel unit took control of Sake, a strategic town near Goma and, while the government in Kinshasa admitted it had lost the battle, it pledged to win the war.
Meanwhile, foreign ministers from nine African countries urged other nations on the continent to contribute troops to fight the rebels. And the UN defended its failed effort to prevent rebels from seizing Goma, saying its helicopters had fired hundreds of rockets at rebels but were unable to beat them back when their forces sharply increased.
UN helicopter gunships flew 17 sorties, firing 500 rockets and four missiles in the defence of the town and two South African peacekeepers were injured.
About 500 rebels made an initial advance, of whom about 64 were killed, but two days later they returned, launching a new attack with 3000 men.