The DR Congo government has rejected an ultimatum by M23 rebels to start negotiations within 24 hours with the insurgents who have closed in on the main eastern city of Goma.
"These are fictitious forces put in place by Rwanda to hide its criminal activities in DR Congo," government spokesman Lambert Mende told AFP news agency on Monday.
"We prefer to negotiate with Rwanda, the real aggressor..." Mende said.
Earlier on Monday, M23 rebels gave the government 24 hours to open peace talks and pull back its forces or risk an escalation of fighting in the country's east.
"To allow for a peaceful solution to the current situation, our movement is demanding that the government in Kinshasa... cease its military offensive... demilitarise the town and Goma airport within 24 hours," the M23 rebel group said on Monday.
The M23 rebel group, who UN experts say are backed by neighbouring Rwanda, on Sunday advanced to within 3 km of Goma, the provincial capital of North Kivu province, after pushing back UN peacekeepers and government troops.
Al Jazeera's Nazanine Moshiri, reporting from Nairobi in Kenya, said: "The rebels are about 3km away from Goma, and only two from Goma's airport. They have been advancing steadily in the last 24 hours.
"Government officials and Congolese soldiers have been retreating. ... UN staff have been asked to stay indoors, international NGO workers and diplomats have been moved across the border to Rwanda.
"Goma feels like a ghost town right now."
Goma airport was closed, a Western military source said, and hundreds of people have fled the city, seeking refuge in nearby camps amid the most serious fighting in the area since July.
Fighting erupted in eastern DRC in April following a mutiny by former soldiers who formed the M23.
The Kinshasa government has accused Rwanda of seeking to destabilise the region and control its minerals resources by backing the rebels.
The rebels say the government has violated the terms of a 2009 peace deal that integrated them into the army, and demanded the government announce direct talks within 24 hours to address their demands.
"The Movement reserves the right to (...) continue its resistance against the government of Kinshasa until its fall" if
the government fails to comply, the statement said.
UN attack helicopters launched sorties against rebels on Saturday as the UN Security Council demanded an end to foreign support for the M23, whose leader was hit by UN and US sanctions last week.
The M23, in turn, has warned UN peacekeepers that "we will respond" if they fail to stop backing the regular army by strafing rebel positions.
"Despite the UN air strikes, the rebels have an advantage. They have night vision equipment and mortars as well. People are expecting the rebel group to make some kind of advance," our correspondent reported.
"The UN is attacking the rebels from the air with attack helicopters, but so far they have not engaged with them on the ground."
In New York on Saturday, the 15-nation Security Council went into an emergency session on the crisis.
Ban Ki-moon, UN secretary-general, appealed to Rwandan President Paul Kagame to "use his influence on M23", according to Herve Ladsous, UN peacekeeping chief.
Rwanda has denied a report by UN experts that it has backed the rebels.