United Nations peacekeepers have started an operation to disarm and drive out Rwandan rebels based in Democratic Republic of the Congo.
Guatemalan special forces and attack helicopters are being used against the militias based in eastern DR Congo.
The presence of the Rwandan rebels has led to years of fighting in the region.
Rwanda has twice invaded DR Congo, saying it is trying to wipe out the rebels. They were supposed to have been disarmed under a 2003 peace deal.
Much of DR Congo's South Kivu region is under the control of the Democratic Liberation Forces of Rwanda (FDLR), accused of responsibility for the 1994 genocide in Rwanda.
"If these armed groups refuse to leave, we will use force to chase them out," said Thierry Provendier, spokesman for the UN Mission in DR Congo (Monuc).
He said that UN peacekeepers and Congolese troops would start Operation Falcon Sweep by meeting militia leaders and asking them to leave peacefully.
FDLR leader Ignace Murwanashyaka has said he and his men are ready to return to Rwanda as long as they are given an amnesty.
But Rwandan President Paul Kagame has refused to give such an undertaking.