With M23 rebels now less than 20 kilometers (12 miles) from Goma, the main city in the mineral rich region, UN leader Ban Ki-moon called Rwanda's President Paul Kagame to "use his influence on M23", said UN peacekeeping chief Herve Ladsous.
Rwanda has denied a report by UN experts that the country has backed the rebels. Ladsous said the United Nations could not confirm whether Rwanda is behind the new rebel offensive but told reporters: "There are reports that the M23 attacking forces are well-equipped and very well supplied."
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The Security Council met in an emergency session demanded by France hours after M23 took another town on the road to Goma. The United Nations set up special protection centers for its staff in the city.
A council statement demanded an end to the M23 advance on Goma and "that any and all outside support and supply of equipment to the M23, cease immediately."
It vowed new sanctions against M23 leaders and those who help it breach UN sanctions and an arms embargo.
The rebels had night-vision equipment which enabled them to launch Saturday's offensive on Kibumba, and have also recently acquired 120 mm mortars, Ladsous said.
UN attack helicopters fired on the rebels at Kibumba in support of DR Congo forces but could not stop M23, mutineers who broke away from the government army in April, from taking the town, which is 25 kilometers from Goma, Ladsous said.
Government forces are now defending Kibati, which is 18 kilometers from the capital of Nord Kivu province. Nearby is a camp of between 60,000 and 80,000 displaced people.
"The fear obviously is that those people will flock towards Goma to seek protection if the M23 advances further," the UN peacekeeping chief said.
"The fall of Goma would inevitably be a humanitarian tragedy, with even massacres and civilian panic," said France's UN ambassador Gerard Araud. "That is why we must stop the M23."
The United Nations and the government of the DR Congo have activated a special security plan in Goma which includes the protection centers, officials said.
There are some 6,700 peacekeepers in Nord Kivu province with some 1,500 troops in Goma and the surrounding area.
The DR Congo government has said 4,000 troops had crossed the border from Rwanda to help the M23. The Rwandan government has strongly denied the claim.
UN chief Ban spoke with Rwanda's Kagame on Saturday "to request that he use his influence on the M23 to help calm the situation and restrain the M23 from continuing their attack," Ladsous said.
He also spoke with DR Congo's Foreign Minister Raymond Tshibanda "to express his full support for the government's efforts to repel the M23," said Ladsous.
Araud said France's Foreign Minister Laurent Fabius had been in contact with his Rwandan and DR Congo counterparts.
The United Nations this week ordered a global travel ban and assets freeze against M23 leader Sultani Makenga, a former army colonel. The US government also ordered sanctions.
The sanctions experts have called for Rwanda's defense minister, General James Kabarebe, to be added to the blacklist, diplomats said.
