A Rwandan-backed rebel group advanced to within 4 kilometres of Goma, a crucial provincial capital in eastern Congo, marking the first time that rebels have come this close since 2008.
Congolese army spokesman Colonel Olivier Hamuli said the fighting had been going on since 6am yesterday and the front line had moved to just a few kilometres outside the city.
Contacted by telephone, M23 rebel spokesman Colonel Vianney Kazarama said the group would spend the night in Goma.
As the rebels moved in, the governor of North Kivu province, of which Goma is the capital, said he had been taken away to the city of Bukavu.
The M23 rebel group is made up of soldiers from a now-defunct rebel army called the National Congress for the Defence of the People (CNDP) which agreed to be integrated into the country's armed forces following a 2009 peace deal.
That rebel group had been led by a Rwandan commando, General Laurent Nkunda, who marched his soldiers to the doorstep of Goma in 2008, abruptly stopping his advance just before taking the city.
Starting in April this year, the members of M23 began defecting from the regular army, claiming that the terms of the peace deal had not been observed.
Numerous reports by human rights groups, including by the United Nations Group of Experts, have shown that M23 is actively being backed by Rwanda and the new rebellion is likely linked to the fight to control the Democratic Republic of Congo's rich mineral wealth.
The latest fighting broke out last Thursday. Two days later, UN attack helicopters targeted M23 positions in eastern Congo, killing two army officers and 151 rebels.
UN chief Ban Ki-Moon has urged Rwandan President Paul Kagame to use his influence and "restrain M23 from continuing their attack".
Reports by UN experts have accused Rwanda and Uganda of supporting the rebels. Both countries, however, strongly deny any involvement.