In a statement to the Security Council, the UN Special Representative in DR Congo yesterday cited "credible reports that the military recruitment of the M23 did not cease" after a December peace deal.
"There are also credible reports of emerging M23 activities in Ituri in northeastern Congo," Martin Kobler said.
"We should tolerate no military reemergence of the M23," he said, referring to the latest incarnation of an ethnic Tutsi rebellion.
The group laid down its arms in early November after an offensive by government troops backed by a special UN brigade.
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And he urged "the governments of Uganda and Rwanda to do everything possible to prevent M23 elements from sheltering or training troops on their territory."
UN experts have accused Kigali and Kampala of giving military aid to the M23, which both countries have denied.
Speaking to reporters later, however, Kobler sought to minimize the danger "M23 is militarily defeated, the ex-combatants have to be integrated."
The UN mission in DR Congo should focus on what he called "our first priority" fighting the FDLR, a Rwandan rebel group that includes the remnants of the Hutu militia which carried out Rwanda's 1994 genocide.
After clearing several FDLR positions, Kobler said, the UN mission in DR Congo is preparing further operations.
"The (FDLR) leadership must be left with no choice other than surrender," he insisted, recalling that this is the 20th anniversary since the genocide.
Kobler also said that "military actions against the ADF are expected soon," naming another armed group active in eastern DR Congo.
Mary Robinson, the UN special representative for the Great Lakes region, told the Security Council "the positive atmosphere" from the December peace talks "has vanished."
She also announced plans to organize "in the first half of 2014." a meeting aimed at attracting private investors to the Great Lakes region.