Tensions have risen in eastern Congo as M23 rebels advanced on Bukavu, the capital of South Kivu province, on Friday after capturing two towns just seven kilometres apart.
The rebels captured Kabamba and Katana, and local sources said they were trying to take Kavumu, just outside Bukavu.
The Congolese army has not commented on the developments, but humanitarian agencies on Friday raised the alarm over the likely humanitarian toll after deadly clashes in the battle for Goma in neighbouring North Kivu left at least 3,000 people dead.
News of the capture of the South Kivu towns came as the Congolese army continued to denounce M23's defiance of leaders of the East African Community and the Southern African Development Community, who have called for an immediate and unconditional ceasefire.
UN Under-Secretary-General for Peace Operations Jean-Pierre Lacroix said the rebels were only a few kilometres from Kavumu Airport, which serves Bukavu, and warned that if they took the airport, it would herald the fall of Bukavu.
He feared an escalation similar to that in Goma and called for diplomatic efforts to avoid a regional escalation and to protect civilians.
He also expressed concern about the risk of escalation beyond the DRC's borders as a result of the spread of fighting in South Kivu.
"If the past is anything to go by, this could potentially degenerate into a wider regional conflict," he warned.
Mr Lacroix cited the tough stance taken in recent days by Rwanda and South Africa, as well as the tone taken by Burundian President Evariste Ndayishimiye, all of whom are involved in the war in eastern Congo.
He expressed concern at a statement by Corneille Nangaa, the president of the AFC/M23, who told a press conference on Thursday that he was ready to "move towards Kinshasa".
