UN attack helicopters targeted M23 rebels in the eastern Democratic Republic of Congo after fighting resumed following a months long lull in violence, a local official said.
Two army officers and 151 rebels were killed in a battle beginning on Thursday that the UN called the worst clash between the M23 group and the military since July.
Attack helicopters for the UN mission in the DR Congo, known as Monusco, had been on standby.
"Monusco helicopters this morning bombarded the M23 positions in the city of Kibumba," said North Kivu governor Julien Paluku.
He said the Congolese army had earlier retreated from Kibumba, which is 19 miles north of Goma, after thousands of Rwandans, who he says were backing the rebels, attacked early Saturday.
"The fighting was very violent between the Congolese military and the M23 rebels backed by the Rwandan army," he said.
"Rwandan forces bombarded our positions in Kibumba since early this morning and an estimated 3,500 crossed the border to attack us."
Reports by United Nations experts have accused Rwanda and Uganda of supporting the rebels. Both countries strongly deny any involvement and Uganda said if the charges continue it will pull its peacekeeping troops out of Somalia, where they are playing an important role in pushing out the Islamist extremist rebels.