The UN Security Council has demanded an immediate halt to the violence in eastern Congo after holding an emergency meeting in New York.
Saturday's session came after M23 rebels, said to have been backed by Rwandan forces, clashed with the Congolese army following a months-long lull in the violence.
The council also called on M23 to halt "any further advances towards the city of Goma".
A regional governor said attack helicopters on standby for the UN Mission in Congo, known as MONUSCO, had "bombarded" rebel positions earlier on Saturday.
The governor, Julien Paluku, said the rebels were backed by government forces from Rwanda.
In New York, Herve Ladsous, the UN under-secretary general for Peacekeeping Operations, said UN officials were in no position to confirm whether Rwanda was directly involved in attacks.
However, he continued: "I would also say that there are reports that the M23 attacking forces are well equipped and very well supplied."
The Security Council also said that it would increase sanctions.
"These sophisticated arms came from somewhere, and it is evident that there is external support"
- Gerard Araud,
French ambassador to the UN
"They [UN Security Council members] expressed their intention to apply additional targeted sanctions against the leadership of the M23, and those acting in violation of the sanctions regime and the arms embargo," said Hardeep Singh Puri, Indian Ambassador to the UN.
He also said members were also demanding that all outside support and supply of equipment to the M23 "cease immediately".
The M23 was created after officers from the Congolese army defected in April and May and launched a rebellion to demand better pay, armaments and amnesty from war crimes.
Direct fighting broke out on Thursday in Rugari, 30km from the provincial capital, Goma.
Gerard Araud, the French ambassador to the UN, added: "These sophisticated arms came from somewhere, and it is evident that there is external support. At this time, we do not have proof to accuse a country or an individual."
The UN and the US have both issued sanctions against M23 leader Sultani Makenga, who is accused of forcing children into M23 ranks.
Another M23 leader, Bosco Ntaganda, is under an International Criminal Court arrest warrant for similar war crimes.
'Cycles of hatred'
Saturday’s clashes erupted hours after the United Nations put its peacekeepers in the region on high alert.
The fighting was the most serious in the rebellion since July when UN attack helicopters were last put into action against M23.
The M23 group broke away from the national army in April, and UN Secretary-General Ban Ki-moon said in a recent report that the rebels now pose a long-term threat to the government.
"Reprisal attacks on civilians are intensifying, fueling cycles of hatred and violence among different communities," Ban warned in the report.
Ban also said, without naming any countries, that he was "disturbed by continuing reports of external support to the M23".
He praised "the strong condemnation by a number of member states of all forms of support to the M23 and other negative forces" operating in the country and called for a halt to "this destabilising assistance",