The 15-nation council issued a statement voicing its "strong condemnation of any and all outside support to the M23 and demand that all support to the M23, including from outside countries, cease immediately."
"They further call upon all countries in the region to cooperate actively with the Congolese authorities in dismantling and demobilizing the M23," the statement said.
It did not name who the foreign supporters were, but envoys said it was clear which countries the council had in mind. Rwanda and Uganda have denied supporting M23 rebels, but council diplomats say privately that those denials - above all Rwanda's - ring hollow, Reuters reports.
"These rebels are very well equipped," a diplomat told Reuters. "They're getting help from somewhere."
Earlier this week, U.N. Congo envoy Roger Meece told the council behind closed doors that M23 had gained ground in the east against the Congolese army, which has only limited capabilities to fight the small but capable rebel group, envoys said.
"The overall view (from Meece) was pretty grim," a senior Western diplomat said on condition of anonymity.
Congo's president, Joseph Kabila, said on Saturday his government had requested an explanation from Ugandan authorities after persistent rumors that its soldiers were backing the M23 group.
The M23 rebellion takes its name from a 2009 peace accord the rebels say was violated by Kinshasa.
It has been swelled by hundreds of defectors from the Congolese army who walked out into the bush in support of fugitive Congolese General Bosco Ntaganda, wanted by the International Criminal Court on war crimes charges.
The leaders of Congo and Rwanda agreed to allow a neutral force to be deployed in Congo to defeat each other's rebels, but the plan's details have not been announced yet.
Uganda is scheduled to host regional leaders from Sunday to Wednesday to discuss the conflict after an interim report by the U.N.'s panel of experts accused Rwanda of backing the rebels.
Kigali has strenuously denied that, and in turn accused the international community of using Rwanda as a scapegoat for the chaos in eastern Congo.
Donors including the United States, Britain, the Netherlands and Germany have suspended some of their financial aid to Rwanda over the accusations that it is backing the rebels.
Although Uganda's relations with Congo have markedly improved in recent years, the two neighbors were not seeing eye to eye in the late 1990s and early 2000s after Uganda, alongside Rwanda, invaded the giant nation twice.
Uganda justified its invasion by saying it wanted to root out a rebel group, Allied Democratic Forces, which has bases in eastern Congo and is opposed to the Kampala government.
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