The government of the Democratic Republic of Congo has expressed alarm as rebel forces advance towards the country's main eastern city of Goma.
Witnesses told the BBC that rebels of the M23 group were 40km (25 miles) from the city, near the Rwandan border.
They said rebels appeared to be taking towns and villages with ease, with government troops usually melting away.
DR Congo has accused its neighbour of wanting to keep it unstable so it can exploit its rich mineral wealth.
The Congolese government has called on the international community to condemn Rwanda.
On Sunday, rebels were reported to have seized the strategic town of Rutshuru, 70km north of Goma.
A senior official at a national conservation park speaking on Monday just 40km north of Goma, told the BBC that "the rebels are very much in control of this area".
BBC international development correspondent Mark Doyle says it is not clear if the rebels intend to attack the city.
If they do, there will be a new and massive humanitarian crisis, he says.
However, the rebels may only be flexing their muscles to strengthen their negotiating position with the government, our correspondent adds.