UN forces have launched air strikes on rebel positions near Goma, the main city in eastern Democratic Republic of Congo, a spokesman has said.
The UN had used “attack helicopters and its artillery” to push back an offensive launched by the M23 rebel group, the spokesman added.
More than 80 people were reportedly killed in fighting last week.
The UN has deployed a new intervention brigade to mineral-rich DR Congo to tackle the rebels.
The rebels seized Goma last November, but withdrew under diplomatic pressure.
The UN peacekeeping mission in DR Congo, Monusco, was fighting alongside government forces, said Monusco spokesman Felix Basse, Associated Press (AP) news agency reports.
“Monusco has enlisted all of its attack helicopters and its artillery… to push back the M23 offensive that is under way right now on the hills of Kibati,” he was quoted as saying.
Kibati is about 15km (nine miles) north of Goma, a city of about 200,000 people.
The M23 said the Congolese army and UN intervention brigade had attacked its forces in areas north of Goma with infantry, air strikes and heavy weapons, Reuters news agency reports.
Government and UN forces have been battling the rebels since last week.
A doctor in Goma, Isaac Warwanamiza, told AP he had seen 82 bodies, including those of 23 government soldiers, on Sunday.
The UN has an 18,000-strong force in DR Congo.
Its intervention brigade, made up of some 3,000 troops, has a mandate to disarm and neutralise rebel groups in the region.
It is the strongest mandate ever given to such a force by the UN Security Council, UN officials say.
The M23 is made up of deserters from the Congolese army.
About 800,000 people have fled their homes since it launched its rebellion in 2012.