A Rwandan-backed rebel group has advanced to within two miles of Goma, a crucial provincial capital in eastern Congo, the first time rebels have been so close since 2008. The advance came despite the deaths of 151 rebels and two army officers in fighting that culminated in a series of attacks by UN helicopters on rebel positions in eastern Congo on Saturday.

A Congolese army spokesman, Colonel Olivier Hamuli, said the fighting around Goma started at 6am on Saturday. He denied reports that Congolese soldiers were refusing to fight and fleeing.

Contacted by telephone on the frontline, Colonel Vianney Kazarama, spokesman for the M23 rebel group, said his forces were poised to capture the city. "We are about to take the town. We will spend the night in Goma tonight," he said. "We are confident that we can take Goma and then our next step will be to take Bukavu," he added, referring to the capital of the next province to the south.

The M23 rebel group comprises soldiers from a now-defunct rebel army, the National Congress for the Defence of the People (CNDP), a group made up primarily of fighters from the Tutsi, the ethnic group targeted in Rwanda's 1994 genocide. In 2008 the CNDP, led by Rwandan commando General Laurent Nkunda, marched his soldiers to the outskirts of Goma but stopped short of taking the city.

In the negotiations that culminated in a peace deal on 23 March 2009, the CNDP agreed to disband and their fighters joined the national army of Congo. They did not take up arms again until this spring, when hundreds of former CNDP fighters defected from the army, claiming the Congolese government had failed to keep its side of the 2009 agreement.

Numerous reports, including one by the UN Group of Experts, have suggested that M23 is actively backed by Rwanda and that the new rebellion is linked to the fight to control Congo's rich mineral wealth.

The UN secretary general, Ban Ki-Moon, phoned Paul Kagame, the Rwandan president, on Saturday "to request that he use his influence on the M23 to help calm the situation and restrain M23 from continuing their attack", according to peacekeeping chief Hervé Ladsous at UN headquarters in New York. He said the rebels were well-equipped, with night-vision equipment.

The governor of North Kivu, Julien Paluku Kahongya, said on Saturday that the Congolese army had earlier retreated from Kibumba, 19 miles north of Goma, after being attacked by thousands of Rwandans, who he says were backing the rebels. "Rwandan forces bombarded our positions in Kibumba since early this morning and an estimated 3,500 crossed the border to attack us," he said.

In Goma, panicked residents tried to get more information on what was happening. A 45-year-old mother-of-five said that she had nowhere to go. "I don't really know what is happening, I've seen soldiers and tanks in the streets and that scares me," said Imaculée Kahindo. Asked if she planned to leave the city, she said: "What can we do? I will probably hide in my house with my children."

Hamuli, the spokesman for the Congolese army, denied reports that soldiers were fleeing.

In 2008, as Nkunda's CNDP rebels massed at the gates of Goma, journalists inside the city reported seeing Congolese soldiers running in the opposite direction having abandoned their posts.

The badly paid Congolese army is notoriously dysfunctional, making it difficult to secure its loyalty during heavy fighting.

Reports by UN experts have accused Rwanda, as well as Uganda, of supporting the rebels. Both countries deny any involvement and Uganda said if the charges continued it would pull its peacekeeping troops out of Somalia, where they are playing a key role in pushing out the Islamist extremist rebels.

The UN security council called for an immediate stop to the violence following a two-hour emergency meeting behind closed doors. The council said it would impose sanctions against M23 rebels and demanded they immediately halt their advance.

"We must stop the M23" because Goma's fall "would, inevitably, turn into a humanitarian crisis", said France's ambassador to the UN, Gérard Araud. He said UN officials would decide in the coming days which M23 leaders to target for additional sanctions.