UN Secretary-General Ban Ki-moon told Rwandan President Paul Kagame on Wednesday he was concerned about UN reports that Rwandan officials were supporting rebels in the neighboring Democratic Republic of Congo (DRC).
Ban phoned both Kagame and DRC President Joseph Kabila to discuss a mutiny in the mineral-rich eastern Congo province of North Kivu, which has been swept by violence since March after hundreds of former rebels defected from the army.
An addendum to a recent report by UN experts found "substantial evidence attesting to support from Rwandan officials to armed groups operation in the eastern DRC".
Rwanda has repeatedly denied the allegations.
"The Secretary-General expressed grave concern over reports that the M23 mutineers fighting Government forces in North Kivu are receiving external support and are well-trained, armed and equipped," Ban's spokesman Martin Nesirky said in a statement.
Ban stressed "the need do everything possible to dissuade the M23 from making further advances and to cease fighting immediately" and urged the Rwandan and DRC presidents to begin talks to defuse tensions between the neighbours.
The M23 insurgents, dominated by Congolese Tutsis, take their name from a March 2009 peace deal that ended a previous rebellion in North Kivu and led to their integration into the national army.
They deserted the government ranks earlier this year, accusing the government of not respecting the agreement.
Like the 2004-2009 rebellion, the current mutiny has its roots in ethnic and political wounds dating back to Rwanda's 1994 genocide.
Later invasions of Congo by Rwandan forces, and Kigali's backing of Congolese rebels, fueled two successive wars that killed several million people.
"WE SHALL CAPTURE GOMA"
Government authorities in North Kivu accused Rwanda on Tuesday of "invading" its volatile eastern borderlands, portraying an advancing insurgency as a Rwandan military operation.
A succession of rebel successes in recent days have once again embarrassed Congo's ill-disciplined and poorly equipped army and opened the way for a possible assault on the North Kivu provincial capital Goma.
Mobs, angered by the rebel advance, roamed the streets of Goma on Monday attacking suspected Tutsis and Rwandans.
The rebels threatened to strike back to protect civilians.
"When we see civilians communities being mistreated ... and the government has failed to safeguard them and (the UN) has failed to protect them, we shall capture Goma," the head of M23's newly established political wing, Jean-Marie Runiga, said on Wednesday in reaction to the attacks.
Peacekeepers from Congo's UN peacekeeping mission, MONUSCO, patrolled the streets of Goma in armoured vehicles on Wednesday and reinforced positions on the city's outskirts.
Meanwhile the Congolese army, routed over the weekend, attempted to reorganise its shattered forces in anticipation of a possible rebel assault.
"We will be consulting with the soldiers to find out what they need and how we can raise their morale. We are ready to take on the enemy," said General Lucien Bahuma, appointed this week to lead the loyalist forces in North Kivu, during a visit to positions in Kibumba, 20km north of Goma.
Soldiers milled around an improvised camp of tents and ramshackle buildings near the front lines as women prepared a humble meal over an open fire.
"I haven't eaten properly for four days now," a middle-aged soldier in a ragged green sweater and pink flip flops said.
"I haven't received any rations since we left Bunagana. Other soldiers are all around here and are in the same situation."