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.
Goma threat
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".
In Goma, a UN spokeswoman said that cars were being damaged and windows shattered in the regional capital as protesters attacked the UN base.
Demonstrators are angry that the 17,000-strong UN force has not better protected them against an offensive by rebel forces.
Over the weekend the rebels, who say they are protecting the area's Tutsi minority, captured a major army camp at Rumangabo and the headquarters of Virunga national park.
A BBC reporter in eastern DR Congo says the main Kibumba camp for displaced people near Rumangabo has emptied, as people flee towards the city.
Gen Nkunda has threatened to take control of Goma.
The UN accused his soldiers of firing rockets at two UN vehicles on Sunday, injuring several troops.
A spokesman for Gen Nkunda denied the rebels were involved.
His rebels attacked Goma last December. Hundreds of them died as the UN used helicopters under its mandate to protect civilians.
A peace deal was signed in Goma between the government and various rebel groups at the end of January.
Although he signed the deal, Gen Nkunda has always refused to disarm while Rwandan Hutu rebels still operate in the area.
About 200,000 people fled their homes after fighting resumed in the area in late August.
The United Nations says many refugees are malnourished and some are dying of hunger.
One-Minute World News
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