United Nations' peacekeepers have been involved in heavy fighting with rebels in the east of the Democratic Republic of Congo as renegade Tutsi general Laurent Nkunda advances on the town of Goma, dpa reported.
Alan Doss, head of the UN peacekeeping mission in Congo (MONUC), told the BBC that UN helicopter gunships and armoured vehicles on Monday supported Congolese troops north of Goma, the capital of North Kivu province.
Tens of thousands of civilians have fled the advance of the rebels toward Goma, which Nkunda has threatened to take.
Aid agencies say well over 100,000 civilians have fled the renewed fighting since August.
"We can't allow population centres to be threatened," Doss told the BBC. "We had to engage."
Angry demonstrators also stoned UN compounds in Goma on Monday, saying that the UN had failed to do enough to protect civilians.
The problems came as the head of MONUC, Lieutenant General Vicente Diaz de Villegas, stepped down for personal reasons after less than two months in the job.
UN Secretary General Ban Ki-moon, who said he was "extremely concerned" by the rebel attacks, Monday appointed Brigadier General Ishmeet Ben Quartey of Ghana to serve as acting commander of the mission.
Nkunda's National Congress for the Defence of the People (CNDP) and other groups in January signed up to peace accords designed to end sporadic clashes that occurred in 2007, four years after the war in the sprawling Central African nation officially ended.
However, the CNDP and government soldiers have been involved in repeated firefights in the eastern North and South Kivu provinces since late August.
Nkunda's troops have until now confined their operations to the east of the nation, which borders Rwanda, purportedly to protect Tutsis from armed Hutu groups.
Many of the Hutus fled to DR Congo after the 1994 massacre in Rwanda, when Hutu militia and military massacred 800,000 Tutsis and moderate Hutus in the space of a few months.
The DR Congo government recently accused Rwanda of amassing troops on the border with a view to backing Nkunda and attacking Goma.
Observers are concerned that the clashes could reignite a wider conflict and plunge the DRC back into chaos.
Over 5 million people are estimated to have died as a result of the long conflict in the resource-rich nation.
The conflict is often referred to as the African World War owing to the large number of different armed forces involved.