Tens of thousands of civilians are likely to be forced to flee their homes during a planned offensive by Congolese and UN forces against Rwandan Hutu rebels entrenched in eastern Congo, a UN agency has warned.

The UN’s Office for the Coordination of Humanitarian Affairs (OCHA) said in planning documents seen by Reuters that attacks on the Democratic Forces for the Liberation of Rwanda (FDLR) would affect hundreds of thousands of people.

The warning came after the UN Security Council approved the offensive against the FDLR, some of whose members were involved in Rwanda’s 1994 genocide, which failed to meet a January 2 deadline to disarm and surrender.

OCHA said the resulting spillover of violence would quickly overwhelm Congo’s weak local capacity, forcing international donors to step in. Around a million people were displaced during the last major offensive against the FDLR in 2009.

Having been at the heart of two decades of violence, an estimated 1,400 Hutu fighters remain entrenched in eastern Congo’s east, where the state has little control. The rebels were accused of targeting civilians in response to the 2009 operations.

After defeating M23 rebels, which numbered more than 5,000 fighters, in 2013, UN and Congolese troops are under pressure to neutralise remaining insurgents in the mineral-rich zone.

Last week, UN and government troops attacked Burundian rebels in eastern Congo in what UN officials described as a preparatory operation ahead of the broader offensive.

OHCA expects at least 368,000 people in North Kivu province and 118,000 in South Kivu to be affected by fighting, while rebels fleeing west into Oriental province could affect a further 90,000 civilians.