Mechanical diggers have spent days excavating the dark, volcanic earth of the city of Goma, preparing long trenches in which to bury the victims of one of the deadliest battles in decades in the Democratic Republic of Congo.

Humanitarian workers in hazmat suits and teenagers in flip-flops and dirty masks tended to the dead amid the overwhelming stench.

“We have days of mass burials ahead of us,” said Myriam Favier, head of the International Committee of the Red Cross in Goma.

Nearly 3,000 people were killed in Goma last week, according to early estimates provided by the U.N. peacekeeping operation in eastern Congo. The fighting between M23, a rebel group that the U.N. says is funded by Rwanda, and Congolese armed forces resulted in the rebels’ capture of Goma last week.

Millions have died in the past 30 years in Congo, where ethnic tensions and fighting over access to land and mineral resources have erupted into several wars. But rarely have so many been killed within just a few days, experts said.

Though most fighting has stopped in Goma in recent days, the city’s capture by M23 rebels has raised fears of a broader war between Congo, Rwanda and their respective allies.