Congo rebels have taken control of the eastern city of Goma after days of clashes with UN-backed Congolese soldiers, a spokesman for the M23 rebel group said.
A senior United Nations source told Reuters that international peacekeepers had given up defending the city after the Congolese troops evacuated.
The rebellion has aggravated tensions between Congo and its neighbour Rwanda, which Kinshasa's government says is orchestrating the insurgency as a means of grabbing the chaotic region's mineral wealth. Rwanda denies the assertion.
A Reuters witness saw the heavily armed rebels walking through the streets of the Goma, past UN armoured vehicles and peacekeepers.
"Despite the attack helicopters, despite the heavy weapons, the FARDC (Congo national army) has let the town fall into our hands," Colonel Vianney Kazarama, a spokesman for M23 told Reuters by telephone.
He said the rebels had left a corridor open for Congolese forces to evacuate.
Goma's capture will be an embarrassment for President Joseph Kabila, who won re-election late last year in polls that triggered widespread riots in Kinshasa and which international observers said were marred by fraud.
The senior UN source told Reuters that international peacekeepers had been unable to mount a defence after Congolese troops evacuated.
Streams of residents headed for the nearby border with Rwanda, saying they had been ordered to evacuate by the army. More than 50,000 people who fled fighting earlier this year have abandoned refugee camps around Goma.
M23 is led by mutinying soldiers who rose up eight months ago, contending that Congo's government violated a 2009 peace deal that was meant to integrate them into the army.
UN experts, however, support the view that Rwanda, which has intervened in Congo repeatedly over the last 18 years, is behind the revolt.
The vast central African nation was shattered by wars between 1994 and 2003 that killed about 5 million people. Many eastern areas are still plagued by violence from a variety of rebel groups, despite UN-backed efforts to defeat them.
The United Nations has about 6,700 peacekeeping troops in North Kivu, including some 1,400 troops in and around Goma.