In four days of battles, the rebels have advanced closer than at any time in their eight-month-old uprising to Goma, the provincial capital of North Kivu and home to the headquarters of the UN peacekeeping mission in eastern Congo.
A spokesman for the M23 rebels said rebel fighters had advanced to within a mile of Goma. "We're just doing this to break the capacity of the FARDC (Congolese army)," he said.
The United Nations has about 6700 peacekeeping forces in North Kivu, with some 1400 troops in Goma and the surrounding area. The force has a mandate to protect civilians.
More than three- quarters-of-a-million people have fled their homes since the latest fighting in eastern Congo erupted in April when a group of soldiers mutinied to form the M23 group.
The rebels accused the government of failing to respect a 2009 peace deal that ended a previous rebellion in North Kivu and integrated insurgents into the national army.
Oxfam said residents were trying to evacuate their families from Goma, while lorries full of Congolese soldiers were seen leaving the city yesterday.
Thousands of displaced people were also abandoning camps in the north of the city to avoid the advancing rebels.
On Saturday, UN helicopters strafed rebel positions, but insurgents have continued to gain ground.
Goma is home to up to one million people, including hundreds of thousands displaced by the conflict.