UN peacekeepers' attack helicopters failed to stop rebels in the eastern Democratic Republic of Congo from seizing another town, as the Security Council demanded an end to foreign support for rebels closing on a provincial capital.
Kinshasa meanwhile made fresh accusations against its eastern neighbour Rwanda, saying it was backing the rebel forces.
A United Nations peacekeeping spokesman said the M23 rebels had taken the town of Kibumba. It lies just 25 kilometres (15 miles) north of Goma, the provincial capital of the strife-torn North Kivu province, which borders Rwanda and Uganda.
The rebel victory came despite the UN's decision to send in attack helicopters to support government troops.
In New York meanwhile, the 15-nation Security Council went into an emergency session on the crisis.
With M23 rebels less than 20 kilometres (12 miles) from Goma, the main city in the mineral-rich region, UN leader Ban Ki-moon appealed to 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 it has backed the rebels. Ladsous said the United Nations could not confirm whether Rwanda is helping the new rebel offensive but told reporters that M23 "attacking forces are well-equipped and very well supplied."
A council statement demanded an end to the M23 advance 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 latest fighting has forced more than 7,000 people to flee to the already packed Kanyarucinya displaced persons' camp, some 10 kilometres outside Goma.
And fears were growing that the rebels might try to take Goma itself.
Saturday's fighting was just the latest clash this week between the army and the rebels of the M23 group, composed of ethnic Tutsi army mutineers.
While each side blamed the other for the latest violence, the UN's peacekeeping mission, MONUSCO, said the M23 had launched an offensive with heavy weapons early Saturday.
As a result, the UN mission had deployed its peacekeepers to protect civilians.
"As part of this, 10 missions were carried out by (MONUSCO) attack helicopters," it said in a statement.
"MONUSCO firmly condemns the renewal of hostilities. It calls on the M23 to immediately halt its attacks, which have caused a deterioration of the already fragile security and humanitarian situation."
The attack helicopters, provided by Ukraine, were put on standby Friday after the M23 attacked the army just north of Goma.
MONUSCO said the latest fighting was taking place about five kilometres from the Kanyarucinya camp, which currently holds between 60,000 and 80,000 displaced people.
The UN peacekeeping spokesman said government forces and MONUSCO peacekeepers "are attempting to hold off a possible M23 advance toward Goma at Kibati," about 20 kilometres to the north.
"As of right now, UN staff in Goma are gathering at security assembly points to ensure their protection," he said, adding that UN forces in the city and its airport are on "high alert".
The clashes are the most serious in the rebellion since July, when UN attack helicopters were last put into action against the M23.
UN experts have said Rwanda and Uganda back the rebels, a charge fiercely denied by both countries.
As the fighting flared, the DR Congo government and army levelled fresh accusations Saturday that the M23 were getting help from Rwanda.
DR Congo government spokesman Lambert Mende said the latest fighting erupted when 4,000 men in columns had descended on DR Congo territory from Rwanda.
Olivier Hamuli, a DR Congo army spokesman in North Kivu province, said the M23 was clearly receiving support from Rwanda.
"I was at the front line myself, and the shots came from Rwanda," he told AFP.
"When our combat tanks come to shell M23 positions, they are coming under fire... from Rwanda."
Hamuli said the army would launch a new offensive Sunday morning "to retake Kibumba and advance on the M23's positions wherever they are."
As the battle raged Saturday, some 50 soldiers' wives burned tyres on one of Goma's main streets to protest the killing and wounding of their husbands, witnesses said.
The M23 rebels are former fighters in the Tutsi rebel group the National Congress for the Defence of the People (CNDP).
The CNDP was integrated into the DR Congo military under a 2009 peace deal, but the mutineers say they rebelled because the terms of the deal were were never fully implemented.