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.
The Congolese government and the UN say Rwanda is backing the rebels, a claim Rwanda denies.
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.
The rebels - who took up arms in April - named themselves the M23 after a failed peace agreement signed on 23 March three years ago.
They are supporters of renegade Gen Bosco Ntaganda, who is wanted for war crimes by the International Criminal Court (ICC).
Gen Ntaganda is an ethnic Tutsi - like the majority of Rwanda's leadership - and a recent UN report accused Rwanda of backing the rebels.
Kikaya Bin Karubi, the Congolese Ambassador to the UK, told the BBC: "The United Nations Group of Experts last week published a report that says clearly that the so-called M23 are using Rwandan soldiers - and that's the United Nations talking, not us."
Rwanda has vehemently denied the accusations.
The M23 rebels defected from the army amid pressure on the government to arrest Gen Ntaganda.
An estimated 200,000 people have fled their homes since April, with about 20,000 crossing the border to Uganda and Rwanda.
Eastern DR Congo has been plagued by years of fighting.
In 1994, more than a million Rwandan ethnic Hutus crossed the border following the genocide in which some 800,000 people - mostly Tutsis - were slaughtered.
Rwanda has twice invaded its much-larger neighbour, saying it was trying to take action against Hutu rebels based in DR Congo. Uganda also sent troops into DR Congo during the 1997-2003 conflict.