UN attack helicopters hit rebel positions in eastern Congo after insurgents gained ground in heavy clashes with government troops.
The clashes to the south of the town Kibumba meant the rebels have advanced to within 30km of Goma.
This is the closest they have been to North Kivu's provincial capital since a rebellion began in the eastern provinces eight months ago.
North Kivu Governor Julien Paluku said the army retreated to the southern outskirts of the town after M23 rebels - a group of soldiers who mutinied in April - advanced with support from neighbouring Rwanda.
A Congolese government statement said 4,000 Rwandans had crossed the border, although Kinshasa later reduced that estimate to 3,500.
Rwanda rejected the accusations, the latest in a string of charges by the Congolese government in Kinshasa.
The Rwandan government called on Congo's army and the rebels to halt the fighting as shells were landing in its territory.
A UN spokesman said they were concerned that the M23 rebels appear to be very well-equipped and supplied.
UN peacekeeping chief Herve Ladsous told reporters that the M23 rebels have sophisticated equipment, including night vision equipment and 120mm mortars.
He said UN Secretary-General Ban Ki-moon had called Congo's foreign minister, Raymond Tshibanda, to voice support for Kinshasa, and Rwandan President Paul Kagame to urge him to "use his influence on the M23 to help calm the situation and restrain the M23 from continuing their attack."