Refugees are fleeing as UN troops move north from Goma
UN peacekeeping forces are engaged in heavy fighting against rebels in eastern Democratic Republic of Congo.
The head of the UN mission, Alan Doss, told the BBC that helicopter gunships and armoured units were supporting the Congolese army north of Goma.
The clashes followed reports that hundreds of protesters had attacked the mission's headquarters, saying the UN was not doing enough to protect them.
At least 20,000 people are reported to be fleeing towards Goma.
The UN is trying to help government forces prevent rebel troops loyal to renegade general Laurent Nkunda from advancing on Goma, capital of North Kivu province.
"We can't allow population centres to be threatened," Mr Doss said. "We had to engage."
The muscular response from the UN shows how serious the rebel advance is, reports BBC World Affairs correspondent Mark Doyle.
Earlier, the UN had said it was considering using helicopter gunships against the rebels, but that government troops were also in the area and could be hit.
News of the UN's engagement came as it was confirmed the military commander of the UN mission in DRC - Spain's Lt Gen Vicente Diaz de Villegas y Herreria - had resigned for "personal reasons" after just seven weeks in the job.
The UN said it was working to "ensure continuity of command and to replace General Diaz as quickly as possible".
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