Dissident soldiers have taken control of the eastern Democratic Republic of Congo town of Bukavu after a week of fighting with regular army troops.
The clashes are delaying the creation of a new army
DR Congo President Joseph Kabila accused Rwanda of sending troops over the border to support the rebels.
The BBC's Rob Walker in Bukavu says the army has withdrawn from the town, which is now calm except for looting.
He says 1,000 United Nations troops around Bukavu have not intervened to prevent the rebel advance.
Under a 2003 peace deal, former rebels should be integrated into the army. The clashes threaten the peace process.
In the capital, Kinshasa, hundreds of people rioted in protest at the UN's failure to defend Bukavu from the rebels.
Demonstrators gathered outside the UN's headquarters and threw stones at UN vehicles, setting one on fire.
UN spokesman Sebastien Lapierre said that peacekeepers were intervening to stop looting.
The dissident soldiers are led by Brig Gen Laurent Nkunda, who has installed himself in the governor's residence in Bukavu.
UN sources estimate that Gen Nkunda has between 2,000 and 4,000 dissident fighters under his control.
Gen Nkunda's fighters are reinforcing Col Jules Mutebusi, whose militia first clashed with the army last week.
Exchanges of light arms and mortar fire erupted on Wednesday at 0400GMT in Bukavu town centre but no gunfire has been heard in the past few hours, our correspondent says.
The fighting came just hours after UN peacekeepers said a rebel ceasefire declared on Monday appeared to be holding in the area.
Rebels say they broke the ceasefire after coming under attack but this has not been independently verified.
A week of clashes has left at least 27 people dead, and thousands of ethnic Tutsi Banyamulenge have fled into neighbouring Rwanda.
But Congolese Foreign Minister Antoine Ghonda has denied there is an ethnic aspect to the violence.
Instead, he blamed it on indiscipline within the army.
"We are in control of the whole of Bukavu and (government loyalist Brig Gen Mbuza) Mabe has fled the city," Col Mutebusi told AP news agency.
"Many of his troops have joined us, others have shed their uniforms and are staying at their homes and a few have fled with Mabe," he said.
Col Mutebusi's militias had earlier agreed to a UN ultimatum to stay off the streets.
The renegade commander himself has been suspended from his post in the national army because of the fighting.
Both Col Mutebusi and Gen Nkunda are former members of the largest rebel group during DR Congo's five-year civil war, RCD-Goma.
A senior government delegation had been expected in Bukavu later on Wednesday, led by former RCD-Goma commander Vice-President Azarias Ruberwa.
Under a peace deal agreed last year, all the Congolese warring factions were supposed to unite as a single army, but progress has been slow.
The deal ended five years of war, which dragged in at least six other countries.