Congolese troops began a military offensive in the eastern city of Beni on Saturday against the Allied Democratic Forces (ADF).
This is a Ugandan rebel armed group blamed for an attack that killed 15 United Nations peacekeepers last month.
The operation is part of a joint effort by Democratic Republic of the Congo and Uganda against the group after the suspected ADF attack on a base manned by Tanzanian peacekeeping troops.
That attack, which also killed five Congolese soldiers and wounded another 53 peacekeepers, came amid a rising wave of violence in the mineral-rich, ethnically volatile area.
"Since this morning, we have launched a general offensive against the ADF phenomena," General Marcel Mbangu, commander in charge of Congo's North Kivu province, told a new conference.
"This is, for us, the final offensive. We will fight them until the end, until we have secured our territory," he added.
Residents reported the sounds of gunfire and explosions in Beni on Saturday.
Rival militia groups control parts of eastern Congo long after the official end of a 1998-2003 war in which millions of people died, mostly from hunger and disease.
A surge in militia violence across the country, which followed President Joseph Kabila's refusal to step down when his mandate expired just over a year ago, has raised fears Congo could slide into all-out war again.
ADF has long been active along the Congo-Uganda border and has been blamed for a spate of massacres. Last month Uganda launched air strikes and artillery attacks on ADF positions on its side.